Tax on Gains from Contract sales

Hi Gurus, please could you kindly advice what tax should be paid in the following situation?

A contract was agreed and exchanged between Buyer A and Developer B to purchase an off-plan property for £300,000 on 03/07/2014. The property is expected to be completed in June 2016. A 10% deposit was paid on exchange, and another 10% paid on 03/01/2015.

Due to a change in plan, Buyer A decided to sell the contract prior to completion. The market has increased during the period, and an offer was received from a third-party buyer C recently to take over the right to purchase under the original contract terms for a consideration of £100,000 (including £60,000 being deposit paid, and £40,000 being premium).

I know the Buyer A will pay no stamp duty tax, and Buyer C will pay stamp duty tax on £340,000 instead of £300,000, but what tax should Buyer A pay on the gain of £40,000? Should he pay capital gain tax or income tax?

I am confused as in theory, A never owns the property, and only has the right to buy at a fixed price under the original contract, so it would hard to argue he had capital gains on something he does not even own. But it also makes no sense to treat the gain as income because it’s once-off, and A has no intention to trade property as a business.

Thanks in advance.