As I have been advised previously, a breach of a condition is a repudiatory breach and Mr A can elect to accept the repudiation and discharge the contract, however, the discharge is for future performance only. So, if the denial of existence of the contract is treated as just a repudiatory breach of the contract, Mr A would be entitled to damages and not to rescission ab initio and restitution.
This was well explained by Lord Wilberforce in Johnson v Agnew , AC 367:
"At this point it is important to dissipate a fertile source of confusion and to make clear that although the vendor is sometimes referred to in the above situation as ‘rescinding’ the contract, this so-called ‘rescission’ is quite different from rescission ab intio, such as may arise, for example, in cases of mistake, fraud or lack of consent. In those cases, the contract is treated in law as never having come into existence. (Cases of a contractual right to rescind may fall under this principle but are not relevant to the present discussion.) In the case of an accepted repudiatory breach the contract has come into existence but has been put an end to or discharged. Whatever contrary indications may be disinterred from old authorities, it is now quite clear, under the general law of contract, that acceptance of a repudiatory breach does not bring about ‘rescission ab initio.‘ I need only quote one passage to establish these propositions.
In Heyman v Darwins Ltd  AC 356 at p 399 Lord Porter said:
". . . To say that the contract is rescinded or has come to an end or has ceased to exist may in individual cases convey the truth with sufficient accuracy, but the fuller expression that the injured party is thereby absolved from future performance of his obligations under the contract is a more exact description of the position. Strictly speaking, to say that on acceptance of the renunciation of a contract the contract is rescinded is incorrect. In such a case the injured party may accept the renunciation as a breach going to the root of the whole of the consideration. By that acceptance he is discharged from further performance and may bring an action for damages, but the contract itself is not rescinded."
So, I am afraid, this approach would not result in rescission ab initio.