We've got two in our group who were in the same position. They've opted to get the Eurostar to Paris and then the train to LM and then return the same way on the Sunday after the race. One did this last year and it was quite a simple journey for him. Both are about the same distance from London as your mate. Of course, it's just a suggestion, but just wanted to let you know what folk in the same situation in our group have done/are doing.
Cheers
Chest