Hi Mate
I have been riding my e-bikes for about 4 years, only ever got stopped once that was for pulling out in front of a cop at a roundabout, he never even noticed the big motor sticking out the side of the bike!
Bottom line is most police would not know if your bikes was legal or not, a simple sticker should fob most of them off, if you havent got one just make one up, the other thing you could do is get it stamped in to the hub with some of those engine stamps.
What you are already doing is the best bet, always wear a helmet make sure you have good lights, the bike is in good working order including brakes and tyres etc, its also a good Idea to have means of turning the power down, I have a potentiometer on my bars which is inline with the throttle, I can turn the max speed down to 15mph which again should be enough to give most cops the brush off.
A problem may come if you are in an accident though however they would have to prove beyond reasonable doubt that it was the motor providing the speed, you take your chances like with anything, I wouldnt advocate using more than the legal limit here in the UK but then I wouldnt advocate using the legal limit :lol:
I also run NIMH as well, F cells, they are hidden though in battery boxes, just be sensible, always peddle and I would bet you never get stopped, I am hoping that the police have better things to do with their time, most e-bikers are very sensible unlike some of the dorks round here on BMX bikes with no lights or brakes.
Can you post some pictures of you bike?
Cheers
Knoxie