CtidBlueMooner said:
There's a massive safe multi storey at Harrow on Hill tube as mentioned.
Anyone thinking of parking at Watford bear in mind the tube station is a good 20 min walk from the pubs and City Centre.
Other stops on the line with big car parks nearby are Northwood and Pinner (huge Sainsburys next door)
The car park at Northwood is only small and by the time you get there will be full of shoppers heading into London. Watford is a good choice. There are a few pubs just outside the station main one called the Flag. Traffic wardens and clampers will be out inforce on a Wembley match day so just watch where you park on the street. A lot of the surrounding areas have residents only parking