Deferred consideration is consideration for the sale of an asset that is (or may be) payable in the future rather than at the time of disposal. It is sometimes also referred to as an earn out.
Deferred consideration may be of a fixed amount and payable on a specific date or it may be contingent on a future event (ie it may not definitely be paid in the future). The amount to be paid may be ascertainable or unascertainable at the time of disposal (ie of a known or unknown value).
True deferred consideration is subject to capital gains tax (CGT) (or corporation tax on chargeable gains) as part of an asset disposal. However, certain structures of deferred consideration are at risk of being reclassified as income rather than capital and so subject to income tax.
There are specific provisions in the Taxation of Chargeable Gains Act 1992 (TCGA 1992) (TCGA 1992 ss 48, 48A, 280) which govern the tax treatment of deferred consideration. Generally, tax will be due by reference to the total amount of consideration due that can be ascertained at the date of disposal, regardless of whether some or all of that consideration is contingent. However, if the amount (or part of the amount) of consideration is unascertainable at the time of disposal then for tax purposes the consideration (or part) received is a chose in action (being the right to receive future payments). This chose in action has its own value, and this value will be used to calculate the capital gain made. A further disposal of the chose in action will occur when the deferred consideration is paid. This type of chose in action is often referred to as a Marren v Ingles right, after the case of Marren v Ingles  3 All ER 95 which established the principle.
HMRC guidance can be found at CG14850 to CG15130.