Thanks for all the replies. I only started cycling again last year, after 10 years of not cycling (or doing any exercise at all, other than an occasional walk) and after a few months of cycling twice a week I was able, on a £200 2nd hand aluminium bike to average 16mph over 20 miles which including hills of 10% where I would be doing 5mph and where I could sustain a 20mph average on the flat for several miles and sprint at 25mph over a short distance.
I am looking at jobs which are 20 miles away and thinking of cycling, but an EAPC would probably be slower as with an extra 7kg of weight I probably wouldn't be able to sustain doing more than 15mph on the flat and so my average speed would be less than 15mph.
One possibility would be to cycle as fast as I could for a short period where maybe I could reach 20mph (bearing in mind extra 7kg) and then rest, pedalling gently where the electric motor keeps me at 15mph, and then repeat process which MAY make me slightly quicker and this would effectively be "High Intensity Training" which is not a bad fitness technique.
But if I am doing 20mph on the flat (and maybe I can get to 25mph) for short periods, then what is the difference in doing 20 - 25mph all the time using electrical assistance - the answer is absolutely nothing from a safety point of view (for me and others), the only difference being I would be breaking a farcical legal rule.
So I am thinking I may make a 250w DIY bike (if I get a job I can cycle to) because I can potentially make it lighter and I have the option of exceeding the 15mph cut-out if my HIT method doesn't work out. I would not set a limit as high as 30mph as I feel vulnerable going down hill at 30mph, so I would not want to be doing 30mph for extended periods, but a limit of 20-25mph, I feel is safe and in the spirit of the law and a perfectly reasonably speed for a normal cyclist.
I like most motorist break the speed limit in a car, but not excessively and if I drive past a load of kids coming out of school where I could legally do 30mph, I will probably be doing less than 30mph, as for me driving and cycling speeds are about common sense and I don't feel guilty about exceeding the laws by a small amount and realistically as with speeding in a car, you rarely get caught as long as you not excessively over the speed limit.
Even by 6 year old daughter can cycle sprint at 12mph on the flat and do more than 15mph down hill, so hopefully the UK law will change some time soon - maybe Brexit will help !