I recall we booked the cheapest advance tickets last time, which was much cheaper than CIV▸ , but as you've highlighted does come with a risk.
CIV gains in the peak ... £41.20 single between Melksham and London International, and 2 month validity on singles. For next Monday ... that compares to £80.50 on the 07:20 (anytime single), £79.00 on the 07:49 (first class advance single) and then £37.50 on the 09:15, etc.