OK .. Headline

£84.10 per person Super off Peak Return Weston-super-mare to London (zones 1 to 6) - total £168.20

Can we do it at lower cost. Assumption being you are travelling up before the show, and home later that same day.

1. There are no DAY returns WsM to London - too far, but if you buy
a) Day return WsM to DID^» (Didcot Parkway - next trains) at £35.80
b) Day return DID to London inc zones 1 to 6 at £33.70
You'll pay £69.50 per person (£139.00 in total).  MUST use trains that call at Didcot.

2. If you spend £30 on a two together railcard, you then get a third off so
* £84.10 x 2 = £168.20 x .66 = £111.00 + £30 = £141.00
* £69.50 x 2 = £139.00 x .66 = £91.70 + £30 = £121.70
Must travel together (so no arguments and returning on different trains)
Railcard valid for a year

3. There is a WsM to London (1 to 6) super off peak day return at £46.00 via Warminster and Salibury.  Slower journey but if you're going to the Festival Hall via Waterloo, could make sense. If you need Paddington, it would be valid from Salisbury with a change at Basingstoke and another at Reading, but be prepared to explain your routing at ticket checks. Railcard discount would apply if you had one, but as a one-off you would only reduce your total from £92 to £90.70 - seems like too much hassle?

4. Advance fares might be available - SINGLES from £18.70, not with travel cards, and you would be very lucky to find that price both ways. You also need to be 100% sure which train you'll be catching back after the show

Errors and Omissions Excepted ... please, readers, let me know if I have missed anything

Thank you for the information Grahame

I suspect that there may be rail works possible but unconfirmed on that day as no times are yet confirmed for some radon, & I suspect that is most likely.

Tweet GWR^▸ (Great Western Railway) & ask when the timetable for that day will be confirmed. I can’t find any website that will offer tickets either, so going to a ticket office & buying paper tickets will be the only way - but they’ll only offer you off-peak flexibke tickets until the timetable is confirmed

Sadly the last bus to Weston is at 6pm, we are seeing a show and it doesn't give us enough time to get back to Victoria

I've stuck the following question on an already existing thread as it's too short term to make it worth starting a new one.

Advanced tickets.  I believe these are available from 12 weeks prior to travel. 
We are going to London on 26 May for RHS Chelsea from Bristol.  Today is on the cusp of that 12 week period.  Is it worth waiting another day to see if cheaper tickets become available?
There's a 5am service from Bristol Parkway at roughly £44 (with Railcard) currently available and I'm wondering if hanging on until tomorrow is worthwhile.

Any info would be welcome.

The 0503 is available at £25.50 w/railcard currently - (super?)off-peak single.

Advances aren't being released anywhere close to 12 weeks out currently as the timetable is still fluid and changing owing to Covid/DfT^» (Department for Transport - about) instructions. I would definitely wait until they are released, and suggest tweeting GWR^▸ (Great Western Railway) to ask when Advances for that date might be released. Or keep[ an eye on the link in the previous post - Advances only available up until the end of this current timetable.

Your date is after the new May timetable, so that needs to also be finalised before they maybe tweakit further if reductions are still required.