If I remember correctly, there is a bike shop, not a particularly big one, within easy walking distance of the Chelsea train station, they had a ton of parts for old school Raleigh Choppers.....
The only trick would be getting a frame custom made, not sure if anywhere in particular specialises in that sort of thing, but have a chat to the blokes at your local Harley shop, and they might be able to put you onto an enthusiast who might be willing to do up a design for you, or even make you up a frame......
No guarantees, but it's what I'd do......