Bit of a minefield this situation.
There would, of course, be Inheritance Tax to pay by your close relative as it's is payable by any individuals who have received money or property as gifts, upon your death (if you die within 7 years of gifting the money).
However under your 'annual exemption' you can give away a total of £3,000 worth of gifts each tax year without them being added to the value of your estate. Any unused tax-free amount will be carried over to the next year and added to the existing threshold. This means you may be able to give a larger amount during that period.
You could You can make regular payments to help with another person’s living costs. There’s no limit to how much you can give tax free, as long as you can afford the payments after meeting your usual living costs.
you pay from your regular monthly income.
These are known as ‘normal expenditure out of income’. and include:
paying rent for your child
paying into a savings account for a child under 18
giving financial support to an elderly relative
If you’re giving gifts to the same person, you can combine ‘normal expenditure out of income’ with any other allowance, except for the small gift allowance.
https://www.propertysolvers.co.uk/articles/how-much-money-can-be-legally-given-to-a-family-member-as-a-gift/
Whatever route you go down I would strongly advise the services of a Solicitor to draw up an agreement.