CG14544 | Consideration for disposal: market value: gratuitous benefit conferred
From HM Revenue & Customs · Capital Gains Manual
The gratuitous benefit conferred on the other party to the transaction need not be a price difference. It could consist of other favourable terms of a contract, for example a very long time to pay the consideration. However, unless the price paid differs from market value, the application of the market value rule will have no direct effect.