Some of the train companies have cheap deals for over 55’s & in the case of Great Western Railway (GWR) > 60 & over. The GWR deal will get you from London (Paddington) to Oxford for £15 return whilst Bath or Cardiff can be done for £30 return. I would try and avoid Fridays as trains can be busy and Sundays are prone to engineering works.
Arriva Trains Wales have a deal of £24 return for 55’s & over. Here is their route map & details.https://www.arrivatrainswales.co.uk/55/Map/
Trans Pennine Express - who operate services in the north of England and up to Edinburgh & Glasgow, also have a ‘Club 55’ deal:> https://www.tpexpress.co.uk/special-offers/club-55/