You can do the deed of covenant for an amount that will bring your parent up to the tax limit before she has to pay tax, you make out the deed for the gross amount but only pay the net amount less 20% tax, as your parent will have no tax liability they can then claim back the 20% tax you paid.
You personally will only receive a tax benefit if you are on the higher rate of tax as the amount the deed is for will be considered not your money anymore so if you paid tax on it at 41% (or whatever the high rate is now) then your parent claims back the 20% amount and you claim back the balance.
I have this running for my father for many years, 3 family member contribute to bring it to the max.