BA. LHR to Grand Cayman single approx £1750
Return approx £820. What is that about?
Although this seems illogical, you are not comparing like with like. Whereas the return fare of £820 is probably (I haven't checked but I'm pretty confident here) a very restricted no-change no-refund advance purchase fare, the one way is probably an unrestricted (or relatively flexible) fare.
That said, yes, it's bonkers, and it is exactly that type of fare structure that the LCCs stripped away and to some extent forced the legacy carriers to drop.