The London bit may be a problem for aircraft due to airspace / restrictions / suitable airfields.
Also around Britain is not really very far for aeroplanes - how would you do each leg and how would you handycap the aeroplanes? For example someone could enter in a 250kt speedster, and someone else in a 90kt slowmobile. I guess you'd have to have mandatory turning points (or a daily course) and have some method of recording the exact route and have each leg designed to take a suitable amount of time. Otherwise the speedy ones would be done in less than a day
I have no idea what would be deemed a suitable entry fee - it depends what is included in the fee I suppose. I'd enter if it was reasonable, as it sounds like a laugh.