I would not be optimistic. Ordinary costs of sale are not deductible when considering what the open-market value of the asset is.
I expect that HMRC would be very reluctant to accept any rescission based on mistake without a court order. Also, in the case you cite Payne vs Tyler the trustees acted on incorrect advice. It is far easier to argue that the act was mistaken in those circumstances, compared to your case where the settlor received correct advise but then for reasons unknown settled more than £325,000.
Also, the fact that no benefit was ever obtained from the trust will be unlikely to assist you, especially since HMRC and the courts tend to be unsympathetic towards trusts and other schemes designed to avoid means-testing.