Yes, returning to Salisbury after the weekend.
Ok, if you are happy to stick to Advances then buying a separate Advance for each day's travel would work out at a total of £50.45:
£20.75 Salisbury to Shrewsbury
£7.55 Shrewsbury to Llanwrtyd
£6.25 Llanwrtyd to Swansea
£15.90 Swansea to Salisbury (splitting at and changing at Newport - £5.65 + £10.25)
These are currently available to buy on virtually any train during the week in question.
Doing it all on a walkup ticket would be a little more expensive, particularly because of the fact that you're 'doubling back' between Craven Arms and Shrewsbury on separate days (for which there is no period return ticket, or Advances).
Hence my best suggestion would be a total of £63.45:
£49.65 Andover to Knucklas Anytime Return, route via Salisbury
£6.90 Craven Arms to Shrewsbury Anytime Day Single
£6.90 Shrewsbury to Craven Arms Anytime Day Single
Your train up to Shrewsbury would need to call at Craven Arms for this to be valid. Almost all trains do, but there are a handful that don't. Otherwise you would need to buy a ticket from the next station south (Ludlow or in the case of the 'WAG Express', Hereford) that your train calls at. All HoW trains stop at Craven Arms.
The first ticket, being an Anytime Return, is valid for unlimited break of journey en-route. The outward portion is valid for 5 days and the return for a month, which is plenty for your intended journey. It is valid both via Craven Arms as well as via Swansea. You would be switching from the outward to the return portion when the HoW train calls at Knucklas.
If you wanted to travel up to Shrewsbury via a different route (e.g. Salisbury-Basingstoke-Wolverhampton-Shrewsbury, or Salisbury-Basingstoke-Reading-Newport-Shrewsbury, or via London) then there would be other tickets which would be more appropriate for your journey.
