OK you can have a throttle as long as it is pre 2016 after that date it will be classed as an illegal bike, but it still must be no more than 250 watts
My etrike is pre 2016 and I have never been stopped or asked to prove it.
Bit pedantic - but if it has a throttle then to be legal it has to comply with ALL the pre-2016 regs to be legal
which includes a max motor power of 200W - not the current 250W
I used to have a pre 2016 ebike and the throttle was a "nice to have" at times but the power differential was noticeable!
as has been said before - chances of anyone stopping you for it are remote and if they stop you for something else then the chances of the copper knowing teh regs well enough to check your bike are also remote
Of course, if they find you doing 40 on a flat road into a head wind while not pedalling then it is rather a slam dunk!
Thing is - if they do specific checks then it is likely to be on a route when a lot of escooter and ebikes are known to be at that time.
Which pretty much means a busy commuting route.
If you are avoiding these routes and riding sensibly then your chances of being stopped are even more remote - if they want to stop ebikes they would be better off concentrating on delivery riders whizzing around on 1000W machine in town centres
so - overall - it is illegal but the chances of getting caught are remote
Unless my moaning to MPs and counsellors has an effect - which it won't