My wife and I live in High Wycombe (HWY) and had tickets for a motorcycle show in London yesterday and had already got my ticket, my wife has a season ticket.
I saw the Chiltern posts about not travelling so we travelled up extra early, on the 06:10 from HWY which comes from Aylesbury, a very pleasant journey on a near empty 6 car 168.
As Jamie above said, many fans drive to HWY to travel to Wembley, either staying overnight as the hotels are cheaper than London or drive down on the day, so we were aware the later trains would be very busy, also many fans go into London during the day before going to the match.
We then travelled back while the match was playing on the 18:02.
Chiltern had some trains starting from Banbury to London, but I guess some fans would have driven there, as it's a short run down the M40, so I didn't want to risk those.