I seem to remember they lay on extra trains for the big games at Wembley Stadium station. No doubt we’ll have more info nearer the time. The good news is a major headache has been avoided with the Brighton to London line running normally.
When I’ve done the return journey for Englang games my preferred route is:
Wembley Stadium - Marylebone - Victoria - Brighton
Rationale...fastest route to Marylebone, then either tube or taxi if running late to get the Brighton train. Plus lots of places to go for a post match meal and drinks...