It's still a tricky question because you can buy exactly the same motor with different ratings, and what's more, many kits come with motors with no markings. Lastly, there's no way to test the rating of a motor, so if it has no markings, it would be difficult to determine what it is unless you 'fess up.
Let's say I ordered a motor that was identical to one in a branded 250w bike, but it's listed as 500w and has no label or it has an easy-peal label on it. Unless a prosecutor was able to go through your Paypal account to find the record of the motor you had ordered and was able to match it to the one on your bike, it would be impossible to prove that the motor doesn't meet the legal requirements; however, there's another requirement that the bike or motor is labeled with the max speed and power or manufacturer of the motor, so you have an obligation to say what it is.
Given all of the above complexities, I would say that as long as you have the correct assist speed limit set, it would be extremely difficult to prosecute anyone for having an illegal motor unless it was stamped or labelled with anything over 250w. Even then, it seems that they're still not interested.
Until now, nobody in mainland UK has been prosecuted for having an illegal pedal bike. That includes the guy who killed a woman while going very fast on a clearly illegal bike.
To answer your question, I've seen lots of instances of BBS02, BBSHD and those 500/1000/1500w direct drive motors used by forum members.
