I have no idea how much it would all cost these days, and have no intention of looking it up, but the longest I've ever done was in FGW days. The first few miles from Abergavenny to Hereford is no longer part of a passenger service, but this diagram cost a fortune back in the day:
Abergavenny-London Paddington via Hereford and Evesham;
London Paddington-Cardiff Central;
Cardiff Central-London Paddington;
London Paddington-Swansea;
Swansea-London Paddington;
London Paddington-Swansea