ReverendFozz and Yew must have read my mind.
This is just the sort of thing that should have been thought of and put into motion ten years ago when the Mk2s were removed from West coast/Anglia Main Line operations.
With today's long distance railway being almost as busy on Saturdays as it is in the week, I doubt there is much stock available to fulfil such an operation, so the idea could now only be fulfilled when further stock gets released from the system (which would be following the introduction of HST replacement I suppose)
A fleet of say fifteen train sets could then provide away travel for:
*Premiership and Championship games where the distance travelled is in excess of say fifty miles.
*All FA Cup ties once the competition is into the last sixteen.
If these fixtures do not provide work for every set then the spares could be used for matches in Leagues 1 & 2, and also for Local Derbies where the train is recognised as a heavily used mode of travel (ie Norwich-Ipswich etc)
One rake could also see use for GB home nation matches in the Six Nations tournament, essentially linking London (KX or Padd), Edinburgh and Cardiff with Twickenham, Edinburgh or Cardiff as appropriate.
Use of the services should be restricted to matchday ticket holders, and prebooked through the organising company. Rakes could be extended/ shortened as required to suit booked loadings and platform capacity at origin and destination stations. In many cases the services would eliminate intermediate stops and the need to change trains en route.
Such a plan would provide work for traincrew and stock on in excess of forty weekends per year, plus the midweek dates, in which cases the services would be most useful as it is often the case, even for short runs, that the last train home leaves about five minutes before the end of the game. It also brings in some track access revenue for use of paths that would otherwise remain untaken.
I can not really see how this would fail. Just needs a forward thinking company and some stock.