If we assume that the ROT buyer has paid £400 of a contractual price of £450, then misses an instalment thereby committing a breach which is stipulated to be one which allows the seller to terminate the contract, and the seller terminates and repossesses, and the goods are worth say £250 so the seller's position is better than its contractual expectation interest, I read Lord Goff as suggesting that the buyer could have a claim in unjust enrichment for £200, that is the enrichment of the seller over and above its expectation interest. The breaching buyer is allowed to sue for failure of consideration/basis but its recovery is limited to an amount which protects the seller's expectation. Notionally the buyer's claim is to recover £400 paid on a consideration which failed, but the claim is reduced so that the seller (as the non-breaching party) has its expectation protected. Or I suppose it could be conceptualized as a counterclaim for breach of contract.

With which principle and/or authority does that conflict? Is it that the buyer had the use of the goods for some time and so the consideration/basis did not really fail?

Lionel