TRAVELLING to / from FISHGUARD

There are no direct trains from London to Fishguard. A change of trains is always required in Cardiff, with sometimes also a change at Clarbeston Road. There are four connections per day with journey times from London varying between 4¾ to 5¼ hours. On Sundays there are only two connections with longer journey times.

As with many long-distance train journeys in the UK, it is almost always cheaper to split the tickets when travelling from afar, and to book ahead.
I bought three: one to London, one from London to Cardiff and the last from Cardiff to Fishguard.

Cardiff to Fishguard, a journey of some 2½ to 3 hours is a bargain at only £10.

Bike reservations are often compulsory on First Great Western trains from London to Cardiff and there is space for only six bikes.
https://www.firstgreatwestern.co.uk/about-us/customer-services/cycling-policy