Well actually the battery improvement is quite fundamental, but unfortunately the motor is too powerful for British law on these things.
I thought 250w was fine in the uk.
All it needs is stickers removing and replacing with lower rated ones if its not legal - no PC Plod will be able to check, + disable the need to peddle - print out a bill of sale pre 1995 from a closed down retailer and Bobs your uncle.
I used to use this trick all the time with Cars, e.g. Remove the make badge and model / Ltr sticker and replace them with another make - park on double yellows if you need to, or overstay in car parks, if you get a ticket the details are all recorded incorrect - Voided ticket. The best bit its not Illegal to tamper with stickers.
So get one of theses and stick a Vinyl Sinclair / Zike logo on it and listen to all the "I remember those crazy Sinclair bikes............"