Per Millett LJ at page 202G in Jervis v Harris  EWCA Civ 9:
The law of contract draws a clear distinction between a claim for payment of a debt and a claim for damages for breach of contract. The distinction and its consequences are set out in Chitty on Contracts (27th Ed.) para. 21 031. As there stated, a debt is a definite sum of money fixed by the agreement of the parties as payable by one party to the other in return for the performance of a specified obligation by the other party or on the occurrence of some specified event or condition; whereas damages may be claimed from a party who has broken his primary contractual obligation in some way other than by failure to pay such a debt.
The plaintiff who claims payment of a debt need not prove anything beyond the occurrence of the event or condition on the occurrence of which the debt became due. He need prove no loss; the rules as to remoteness of damage and mitigation of loss are irrelevant; and unless the event on which the payment is due is a breach of some other contractual obligation owed by the one party to the other the law on penalties does not apply to the agreed sum. It is not necessary that the amount of the debt should be ascertained at the date of the contract; it is sufficient if it is ascertainable when payment is due.