I have been having the same problem too on domestic flights on some dates.

If it is of any help to anyone, it appeared that it was bookable via the avios.com website, although the price points beyond the 50p rate seem to be slightly higher on there (e.g. £20 + 5250 avios instead of £17.50 + 5250 avios).

Apart from a slightly higher price I suppose the downside is I don't think you can cancel online, and I think I recall someone reporting previously that they will charge you the full £35 to cancel rather than just keeping the Reward Flight Saver cash element if lower.

I have no recent direct experience of using avios.com for BA flights since they stopping having the the extra G class availability many years ago.