taken from the HM Revenue & Customs site
Goods sent as a gift are not free of import duties and import VAT. However, customs duty will not be collected if the amount is less than £7, and under EC Directives and Regulations excise duty and import VAT is not chargeable if:
the value of the gift does not exceed £36
the customs declaration is completed correctly
the gift has been sent from a private person outside the EU to a private person(s) in this country
the gift is for the use of either yourself or your family
there is no commercial or trade element and the gift has not been paid for either directly or indirectly
alcohol, tobacco products etc are within the allowances detailed in paragraphs 2.5 and 2.6
the gift is of an occasional nature only eg for a birthday or anniversary.