Hello DRAV, here is my long-ass spiel about my bike, which seems like it will be similar to what you want.
Okay, if you are set on the Nyx style frame (Qulbix Raptor 140 is what I have, they have 140mm wide and 76mm wide frames), that's great.
I went with Qulbix over Nyx because if I damage the frame, it's steel and can be easily fixed to essentially new condition with a bit of welding/grinding and some paint. Composite frames are awesome, they are much lighter, but they don't fail the same way as metal frames. They crack and fray. I don't know if they can be repaired the same way as steel frames can be. Maybe they can though. I also, personally, feel more comfortable working with metal, drilling through it, etc... Another benefit to the Qulbix frames is you can get the Moto seat, although its very... conspicuous, lol. Technically you can buy almost everything from Qulbix, but I am not partial to some of their parts (DNM suspension is actually made in china by one company, and labelled by multiple manufacturers, one being DNM), and its cheaper to get certain things from other stores. That said, beyond the frame, the only other Qulbix parts I own are the front and rear fenders. Hard to find a front fender for USD/inverted forks, and same with a fender for the rear swing arm.
For the motor, I would suggest QS. I don't even know if Cromotor still does mods to QS motors, but there are accessories you can get for the QS that are made for the stock version, that let you run higher power or just the same amount of power for longer periods (Statorade and Hubsinks), I don't know if they would work with a Cromotor. Another benefit of QS is that you can now order them laced and trued from factory. If I was you, I would get the QS205 50H V3. You can get it with various windings, but the most popular are 9.03Kv(5T) and 11.39Kv(4T), colloquially referred to as torque and speed, respectively. That is, at the same voltage, the 9.03Kv will putout more torque than the 11.39Kv, but the 11.39Kv can go faster at that voltage, its a bit misinformed, but a good general rule of thumb.
So yeah, I would get the 9.03Kv motor, I got the 11.39Kv and I regret it, there is no need for that top speed, and having more torque capability at more normal speeds is much more beneficial, imo. The motor is USD276, it has been shown to handle 14kW of power peak, and I would get QS to mount it in a 19x1.6" rim, this all depends on what tires you want to run as they might not have the right size, 18x2.15" is the other rear tire option I would choose. Then you can also get the 19x1.6" front wheel. Front and rear lacing/truing together, adds an extra USD187.90, which is a good deal, unless you want to true/lace yourself. You can also run the same tire front and back with the 2 19x1.6" wheels. This takes care of the motor, rims, spokes, and front hub.
Adaptto controllers are very nice, a well thought out and integrated system, you get the controller, the interface, you can also use the builtin BMS and get a charging coil, so its an all in one system. You can also buy a 2nd controller for a 2nd motor and daisy chain the the controllers, with one main controller controlling the other, so you can have a proper 2WD system. I have the Midi-E, would have gotten the Max-E, but they were sold out and backordered for months when I was purchasing ebike stuff. If you are going to run a front hub motor, you will need beefy front forks. I have Manitou Dorado Expert forks, and I would say they are on the lower end, in terms of tube diameter (36mm), of what I would use for a high power 2wd system, simply because of the torque, bicycle forks aren't meant for those kinds of stresses, so the beefier the better.
As for the front/rear suspension, just go over to Chain Reaction Cycles and browse, that's what I did, they have great prices, I also used JensonUsa.com. I live in Canada, and I still used CRC. I wanted USD/inverted forks, as that is what I had on my DRZ400SM, and I loved those forks, I ended up with the Manitou Dorado Expert, as they were priced well at the time, prices seem to have gone up a bit (about $200), but they are great forks. I also got a Cane Creek DBcoil rear shock. I went with coil over air because I understand coils coming from dirt bikes/dual sports. I can't say if either of those products is still for sale, but there are most likely lots of comparable products. I tend to buy moderate to high end parts.
For brakes I bought Magura MT5e brakes, specifically meant for eBikes, they have an integrated switch. I then bought some HIGO cables, as that is the electrical connector that Magura uses, from Reinhard at e-bike-technologies.de, so I could run those cables to the adaptto controller for ebraking. I need to fiddle around with this setup now, since the adaptto can now do variable ebraking, I might get a thumb throttle to do that now. But I still want the added safety of the motor cutting off when I pull the brake levers.
What else is there, gearing will be limited as QS motors are pretty big. With the largest dropout width swing arm (165mm, what I have) on the Qulbix, I can fit a 5 speed freewheel on the QS motor, with the smaller swing arm (155mm), I think you can only fit a 3 speed. With the Nyx frame, it has a 150mm drop out, so you might have to go with a 2 speed or single speed freewheel. But you can also mount a Hammerschmidt or a Schlumpf front crankset, they both are 2 speed internally geared cranksets. The Hammerschmidt uses a handlebar gear selector, the Schlumpf uses a foot activated button on the crankset. Qulbix requires that you notify them first if you are getting a Schlumpf, as they need to chamfer the bottom bracket to fit it. I have a Hammerschmidt AM, it comes stock with a 24 tooth chainring, so you get 24T or effectively 38T (w/ 1.6x). But I designed a modified chainring, there is also a guy on here that sells them, for my hammerschmidt that lets me mount a larger chainring on it, so now I run a larger 38T chainring, and I get 38T or effectively 61T.
As for batteries and BMS, the adaptto can take care of the BMS stuff if you buy the extra BMS board. The batteries I can't say, it's up to you really. It's hard to ship large numbers of cylindrical cell batteries to Canada, so while I want to run 26650 batteries, as they sit perfectly inside the Qulbix 140 frame. I'm currently only running a small number of LiPo's until I can figure out how to get those 26650s I want.
Note: I agree with MadRhino, tell the person once that the bike might be illegal in their jurisdiction, and then leave it at that, no need to repeat it over and over. After that, all we should be doing is helping them build a bike that will be safe with the power requirements they want. For instance, I would never recommend putting a QS205 in a regular bike frame, that is unsafe.
Personally, I don't agree with regulating ebikes by motor power, it realistically doesn't denote any metrics or specifications about the motor or bike except how much power the motor uses at peak efficiency, which is useless for safety purposes. It doesn't tell you anything about speed, acceleration, or torque. They should only regulate speed, just like they do with cars and motorcycles. Here in Canada it should be 32kph anywhere on public property (roads, parks, etc...), that way people legally don't need 2 bikes if they want more power in private or in unregulated areas. Drop the 500W and under requirement.
With that said, unless you want to go the civil disobedience route, I would pickup a sticker that says (xxxW) for your motor, Holmes Hobbies sells 500W ones, but I don't know what the ratings are for the UK, I saw 250W bandied about?