As someone who no longer flies regularly enough to build miles that way; the easy answer to this is don’t concentrate on a single program.

Switch your credit card/ points earning to AMEX / another agnostic points builder and build points that way.

There are often good sign up bonuses and then also good transfer bonuses (although the latter less common in the U.K. it seems).

You can then transfer the points to where there is value/ availability when you need them.

I’ve redeemed on BA, VS, LH (via Air Canada), AF and DL over the last few years, having only earned miles on DL. All other redemptions have come via AMEX, supported by a bonus offer and more importantly availability too