White Waltham is the best airfield but Wycombe Air Park is easy to get to - you get the train from Marylebone direct to High Wycombe or from Paddington to Marlow and then its a short taxi ride. Try googling the following:-
Wycombe Air Centre
British Airways Flying Club
West London Aero Club (White Waltham)
Someone asked about using an FAA IR in the UK, I believe you can if flying in a N reg, otherwise you can get your FAA IR converted to a CAA IMC. But be aware that in the UK you get charged not just for landings but also for instrument approaches. Even at small airfields you can pay £15-£20 for each.