As already mentioned, you can do it cheaper by splitting the tickets. Looking at today (Thursday), the £180 return fare only applies to early departures from Paddington. If you turn at Paddington & ask for an OFF PEAK RETURN To Bath, it will cost you £69 and will apply to all trains after (& including) the 8.30am departure. You can use any return and the journey takes 1 hour 30 minutes.
You may also find that the return fare drops to £55 if you leave Paddington on the 10.30am (or later) Monday > Friday. On weekends, you are likely to find the lower £55 fare applies to all trains. The train company for the Paddington to Bath route is www.firstgreatwestern.co.uk
If you buy at Paddington a ticket to Swindon (OFF PEAK RETURN) = £55 plus an OFF PEAK RETURN from Swindon > Bath = £10.70, the total will be £65.70 return.
Going for a 3 way split - all as off-peak returns > PAD > Didcot = £29.60; Didcot > Swindon = £21 and Swindon > Bath = £10.70. So, we have now got the fare down to £61.30 return with the only rules being that you must depart PAD on or after 8.30am & the train must stop at the split points. You can find this out at www.nationalrail.co.uk by putting in for the journey and clicking 'Detail' > 'Show Calling Points'. The 8.30 departure does call at Didcot but the 9am does not. It goes on like this all day (I think) with hourly departures not stopping at Didcot whilst those on the half hour do. On the return, it looks like those leaving Bath at x13 past the hour do not stop at Didcot but those at x43 minutes do.
Cheaper fares can be found by booking in advance. Such fares will tie you down to specific trains.
If you want to use the cheaper but longer option from London (Waterloo), you must select 'other options' and put via SAL (Salisbury) or it will simply direct you to the dearer Paddington trains. Anyway, paying today & going today from Waterloo to Bath will be £40.40 return with a journey time of 2 hours 44 minutes. You must say that you want a Waterloo to Bath off-peak return VIA SALISBURY. A direct train leaves Waterloo at 9.20am to arrive Bath at 11.46. This train will split at Salisbury with one half going to Exeter. Make sure you board the Bath & Bristol bound section. A direct train back departs Bath at 16.07 to arrive at Waterloo at 18.49. (You can also depart Bath at 16.35 or 17.36 on the Cardiff > Portsmouth trains & get off at Salisbury to switch to a London (Waterloo) train. (It does not matter that the Cardiff > Portsmouth trains are a different company).
You may want to ask the staff at Salisbury if you can leave the station to see the Cathedral = about 13 minutes walk.