Some arguable possibilities might be tentatively suggested:
What if the agreed rate of interest was lower than the interest which would be awarded if the claim was based upon a failure of consideration?
What if the English court had no jurisdiction over a contractual claim?
What if different systems of law applied to the contractual and restitutionary claims?
What if B is a minor and cannot be sued in contract?
What if the obligation is to repay in a different, now devalued, currency?

Some very tentative suggestions on Andrew's point,
i) In return for B promises to pay C £5 on demand, A pays B £5. B refuses to pay C. A should have a claim for £5 based upon a failure of consideration. Should it make any difference that B's promise is to pay £5 to A rather than C?

ii) However, The Trident Beauty might be authority against any restitutionary claim. A owes B £5. B assigns his right to payment to C. A pays C £5. B fails to perform his side of the deal and comes under an implied contractual obligation to repay A. A has no restitutionary claim against C, and he is limited to his contractual claim against B. Surely the same would be true if there was no assignment?

I have never been very happy with the Trident Beauty.

Robert Stevens