500W rated motor and full throttle is what it is - a moped. Why make an effort to disguise it as a bike? I think a new moped class would be a much better solution with a quick and simple registration.
Really? Raleigh moped at the beginning of the 1960s had power outputs of 1.3bhp to 1.7bhp. That translates to 969 watts and 1267 watts, not 500. These were VERY conservatively powered machines and modern 'moped' class bikes are limited too 2.5bhp or 1864 watts.
I was amazed at some of the hyperbolic 'danger' talk in the press about increasing average power to a max of 500 watts. There was talk about these bikes being 'dangerously heavy' and powerful. An idiot on any kind of bike, or even on roller blades is a danger if he is travelling fast and on a pavement. It is not the 500 watts or 250 watts that creates risk. It is the idiotic among the population who do that. Th bracket also includes some runners and some in human powered cycling.
I live up in the South Tyne Valley near Brampton and Haltwhistle and the hills around here are killingly long and steep. I can ride them with my 250 watt, Bafang crank motor, but it is a matter of bottom gear, a lot of hard work and 3 to 5 miles an hour. By the top - sometimes three quarters of a mile with 1:15 and 1:20 slopes in part, I am breathing like a steam engine.
EDIT: Wikipedia says that electric mopeds are allowed up to 4KW motor power or 5.4BHP.