CGT may not apply at all. You may be liable to income tax instead if the intention on acquisition is to sell for a profit; see Condition A in para 4. You only need to be within one of these conditions to be caught. The rules are within Income Tax Act 2007 and override the CGT treatment. They deem that you are carrying out a trade - ie a deemed trade, not an actual trade so you cannot shelter the disposal profits or capital gains by making pension contributions.
Note that under 2(a) to (c) your wife can also be taxed on a proportion of the profits as a 'provider of the opportunity'. As this is a deeming provision, it may not affect her need to financially exit the property before March 2026, unless she need to suppress her level of statutory income.
517BDisposals of land in the United Kingdom
(1)Section 517C(1) applies (subject to subsection (3) of that section) if—
(a)a person within subsection (2)(a), (b) or (c) realises a profit or gain from a disposal of any land in the United Kingdom, and
(b)any of conditions A to D is met in relation to the land.
(2)The persons referred to in subsection (1) are—
(a)the person acquiring, holding or developing the land,
(b)a person who is associated with the person in paragraph (a) at a relevant time, and
(c)a person who is a party to, or concerned in, an arrangement within subsection (3).
(3)An arrangement is within this subsection if—
(a)it is effected with respect to all or part of the land, and
(b)it enables a profit or gain to be realised—
(i)by any indirect method, or
(ii)by any series of transactions.
(4)Condition A is that the main purpose, or one of the main purposes, of acquiring the land was to realise a profit or gain from disposing of the land.
(5)Condition B is that the main purpose, or one of the main purposes, of acquiring any property deriving its value from the land was to realise a profit or gain from disposing of the land.
(6)Condition C is that the land is held as trading stock.
(7)Condition D is that (in a case where the land has been developed) the main purpose, or one of the main purposes, of developing the land was to realise a profit or gain from disposing of the land when developed.