I am pretty sure that all electric bikes sold in the uk are supposed to be pedal assist otherwise they would be classed as electric motorbikes. There are exceptions that I’ve seen, I guess the regulations are relatively slack.
As others have said the good ones are not cheap. The main thing to look for should be dealer support, manufacturer support and back up etc. The cheaper ones are basically tat and very prone to failure. Even the more expensive ones can be tempermental but you should be able to take it back to where you bought it they should be able to diagnose or fix the problem. If you buy a cheap kit off ebay or a budget job what’s going to happen when it invariably develops a fault? You’re left with a 70lb piece of sh*t that no one wants to bother trying to fix.