Hi,
There are no Income Tax implications on the receipt of a cash gift unless the cash gift generates interest or dividends, so it should not be declared in a self assessment tax return. These would then potentially be subject to tax.
Further guidance can be found here:
Tax on savings interest
Tax on dividends
If the gift is from a resident in the UK, they may wish to consider inheritance tax implications, that may arise from the gift.
They may wish to review the guidance below for more advice.
How Inheritance Tax works: thresholds, rules and allowances
Thank you.