Check the times on the plate - Bank Holidays are not always exempt. For Brentford on the 26th you need to see something like '8am - 6pm Mon-Fri' or 'Except Bank Holidays'
This is true, and if memory serves me well we sold out (or very nearly did) our allocation for the FA Cup match in January but there was a fair bit of available space to stand
The District and Circle lines have closures on Boxing Day too. I think the best bet is Victoria to Boston Manor (change at Green Park) then a 30 minute walk to Griffin Park (or bus I suppose)