If your staking activity does not amount to a trade, the pound sterling value of any tokens awarded will be taxable as income (miscellaneous income), with any appropriate expenses reducing the amount chargeable.
The charge is on income arising in the tax year; this is normally the year when the income is received or made available to you
HMRC's position on valuation is set out here:
BIM100150 - Miscellaneous income: calculating the profits
CRYPTO21000 - Cryptoassets for individuals: Income Tax: contents
Reasonable care should be taken to arrive at an appropriate valuation for the transactions using a consistent methodology.
Details of the valuation methodology should be kept.
As stated above, the sterling value of the tokens at the date of receipt (or date made available) will represent the taxable income, so it would be reasonable for this sterling value to represent CGT cost.
CRYPTO21200 - Cryptoassets for individuals: Income Tax: staking