There is no HMRC guidance on the tax treatment of crypto referral income, so you would need to examine the details to determine the tax treatment.
Referral income is most likely taxed as miscellaneous income and will only in exceptional circumstances amount to a financial trading activity.
Where the referral income does not amount to a trade, the sterling value (at the time of receipt) of any tokens received from referrals will be taxable as miscellaneous income subject to income tax, with any allowable expenses reducing the amount chargeable.
If the taxpayer is a ‘financial trader’ in cryptoassets this income may be treated as trading income rather than miscellaneous income.
Capital Gains Tax
The sterling value of the referral income at the date of receipt is the capital gains tax acquisition cost of the tokens; to be deducted from the proceeds of a later disposal of the tokens (which is subject to capital gains tax).