Where is the UK citizen resident? Are the two people married or family members? What's the purpose of the gift?
As a general rule and if we assume the person is resident in UK, gifts are tax free up to 3,000 per year. IIRC, there is a way to carry over an exemption from the previous year to the next year, meaning you might be able to give up to 6,000 GBP provided no gifts were received by the recipient the previous year.
As always when a question is very specific to a jurisdiction's tax laws, speak with a tax adviser/lawyer if you want certainty.