Hi Adamski,
You can buy most Chinese bikes, an Oxygen bike, a Juicy bike, a Wisper bike, a Cyclotricity etc and have a throttle fitted. There is no legal issue with having a throttle that lets you ride the bike at 15.5mph. Just order it with your bike.
The law forbids manufacturers to sell an electrically assisted pedal cycle with a twist and go throttle. We manufacturers can fit a throttle and activate it after the user pedals a few seconds. That's the most common method because it only involves a little software change. The second method is to fit an after market throttle. The first method is the most popular because it's legal, the second less popular because it relies on a DfT concession, not the same certainty as the law.