Big Ban Vader, Waterloo to Fratton seem unaffected. It’s the other route to Portsmouth that appears to have the replacement buses.can check on southern rail website but also if you look at buying tickets from Waterloo to Fratton on Trainline.com there’s no mention of any disruption. However, for comparison if you try waterloo to portsmouth you will see it mentions a bus replacement. I’m booked on the 11.09 Waterloo to Fratton.
Basically Fratton normally has three different routes to/from London as follows:
a) Southwestern railway via Guildford to Waterloo (this is known as the Portsmouth Direct Line)
This is normally the fastest service and has 3 trains per hour with varying stops and times.
On the date of the Alty match this line is closed with a bus replacement between Guildford and Petersfield.
b) Southwestern railway via Eastleigh and Basingstoke to Waterloo. (this is known as the South Western Mainline)
This is basically fast to Woking then a stopping service via Basingstoke, Winchester and turns off the mainline at Eastleigh to serve a Branch to Portsmouth.
This line generally only has one train per hour. Journeys are around 30 minutes slower.
On the day of the Alty match this line is open throughout.
c) Southern Railway via Chichester and Gatwick Airport to Victoria. (this is known as the Arun Valley Line).
This line generally only has one direct train per hour and is also around 30 minutes slower but usually has cheaper tickets. Its also good if your journey beyond London is by National Express Coach (as the coach station is about a block away from the railway station).
On the day of the Alty match this line is closed with a bus replacement between Havant and Barnham.
So basically that day there are direct trains between Fratton and London but its a matter of making sure that you are on a train that goes via Basingstoke and Eastleigh and that you allow enough time for your journey and take account of the fact it is likely to be a reduced frequency.