Of course not. It should be about £42 each way.
You are going against decades of entrenched fares policy.
Back in the 1960s "the fare" was distance based, and gave you an Anytime ticket.
BR even published a single page in the timetable with fares against distance.
That (adjusted for inflation etc) fare is still believed by the industry to be "the fare", and any other fare at a discount comes with restrictions.
We now have all sorts of intermediate fares, with the lowest being only about 10% of "the fare".
Normal folk got to believe that the time-limited Off Peak fare (typically 40% discount) was "the fare", when it wasn't.
Now folk seem to think that the one-train-only Advance fare (typically 75% discount) is "the fare".
Only about half of the tickets are regulated, the rest is down to the individual TOC.
There is considerable confusion about which tickets are regulated.
For Cardiff-Edinburgh I think it will be the Off Peak Return.
I agree with you about Off Peak Singles being only £1 less than the return.
This was again BR policy and is unchanged after 20 years of privatisation.
To cut the single to 50% of the return, the DfT says, would undermine the whole basis of the Anytime fares structure.
Virgin WC will sell you a discount single ("half-saver") if you also book a fare the other way as well. It has to be their fare, and on their web site.
GWR has something similar.
Actually a 50% off-peak single would put it in line with most air fares these days.
Time was when BA followed the BR pattern and let you travel off-peak cheaply on a return basis (Saturday night stay), but forced you to pay full whack for one way only.
Airlines like Lufthansa still do it.
The DfT had an investigation into the fares system a few years ago, and promised changes, while at the same time robustly defending the status quo.
Basically they won't make changes which risk reducing overall ticket revenue.
So far, changes have been glacial, concentrating on smart ticketing and "shoulder" peak fares.
This was the consultation and their response to it:
https://www.gov.uk/government/consultations/rail-fares-and-ticketing-review