There have been American Football games in England before...two teams played an exhibition match at White Hart Lane years and years ago...but as for interest in the UK...were are the Scottish Claymores and London Cardinals now? Wasn't much interest in them...
As for the stadium, Twickenham will be a no go in November due to Internationals and there are very strict agreements as to how many times Twickenham can be used each year...