I'll just quote this public exchange ...
Why has the 7:43 fast train from chichester to brighton stopped? The only fast train on this line and now gone. I hope there is a good reason for this
Reply2h
[name] this was a DfT» directive to reduce duplication of services, it also enables GWR▸ to add more carriages to busier services on other parts of their network, particularly the Cardiff/Bristol to Portsmouth route.
Reply1h
[name] I get that. Just now means there are now only 2 very slow and busy trains between portsmouth and brighton now peak time. Which is more a southern issue than gwr