If I buy a moped or motor bike I can't ride it on dedicated cycle tracks, riding a over powered motor bike on the public road is not really a problem except for the lack of insurance, it may be technically against the law, riding with no insurance or MOT to ride on the public road, but it is riding on shared pedestrian/cycle tracks where the problem lies.
It states not over 250 watt on a continuous basis, so a motor that peaks at 350 watt may be within the law, but 750 watt is clearly going OTT, but unless the gearing allows you to pedal that fast, even a 1000 watt will not allow most people to reach 20 MPH as they simply can't turn the pedals that fast, but add a throttle and then yes the user can ride at silly speeds.
But with a mid motor starting off at junctions is not a problem, motor cuts in fast, but with my e-bike with motor in real wheel without a throttle junctions are dangerous as the assistance takes so long to kick in, so one needs a throttle or similar, but at 4 MPH the non pedal assistance should stop, however there is no option for the user to program this in. Either the throttle works at all speeds or is disabled, there seems to be no option for the user to configure his bike to comply with the new laws.
It was permitted to have an active throttle, it depends on the build date, but my e-bike does not show build date.
Had my bike be installed with a 750 or 1000 watt it is reasonably clear that a 250 watt hub is very much smaller than 750 watt so no bike shop is likely to make an error and sell a 750 watt in error, but the 350 and 250 watt hubs are harder to tell with a quick look, need to see the wattage marked on the hub.
My
shown here I got second hand when the DVLA failed to renew my licence in the 2 months I had allowed, it took them 8 months to renew my licence, it has the lowest wattage motor option, and my wife's Bosch mid motor bike out preforms mine, so even if it says 350 watt on the hub, it is not as powerful as the 250 watt Bosch mid motor, it is poor at hill climbing, once the speed drops below 4 MPH may as well give up and dismount.
But technically likely not permitted even if the continuous power does not exceed 250 watt. However there is nothing in that advert that alerts anyone buying it that it likely does not comply with British law, I thought I was getting a fully compliant e-bike it was much latter I realised it said 350 watt on the motor.