I'm a veteran of leccy bikes
Got my first one in 2008 - which I still use weekly.
I also converted my mountain bike for offroad leccy use (1000w), and have a little folding leccy bike that is great for commuting and keeping in the campervan.
As others have pointed out, the time of 2000 quid leccy bikes was 6-8 years ago. there is no reason why a good leccy bike should cost any more than a good bike + 400 quid for motor/battery.
There are lots of kits out for DIY, and there are quite a few companies who will sell you one ready to ride in the UK.
Or of course there is the cheap chinese jobs which are ok if you just want an occasional electrified bike based on a <100 quid frame. Horses for courses.
My 6 year old one is a Trex 7.2FX by cytronex. Frankly I wouldn't recommend a cytronex, as I have since replaced all electrical parts with my own (new hub motor, wiring, controller and my own lipo) after numerous techincal issues. But the idea of starting with a good hybrid bike is definately the best approach imho.
As others have stated the UK law is every so slightly more sensible than EU law in not requiring 'pedalec' only mode - which is just bollocks. I like to let the motor work on its own sometimes on straights/uphills, and then other times, I put effort in. that's the way I like the ride - WTF point is there in forcing pedalling - it's moronic.
The 15mph is also far too low imho. If it was more like 20-25mph I think we'd see a bigger uptake on electric bikes - power to weight (inc the rider) they are the best suited form of transport for electrifying - and yet it's held back by moronic laws like 250w/15mph/pedalec(EU).
fat and live in a hilly area ? tough shit - your 250w motor won't get you up the hills and will pull you along at walking pace. what the hell sort of intensive is that to people.
There seems to be no logic whatsoever to the 250w rule any more than the top speed or EU pedalec thing other than to protect incumbent industries as far as i can see.
Also - remember that most companies offer the bike scheme now enabling you to save tax, etc.
stu