From HMRC guidelines:
2.2 Do I have to pay import duties and/or import VAT on goods sent to me?
Most goods arriving in the UK from outside the EU are liable to any or all of the following taxes:
1. customs duty
2. excise duty
3. import VAT
and must be paid whether:
1. you purchase the goods or receive them as a gift
2. the goods are new or used (including antiques)
3. the goods are for your private use or for sale.
2.3 Are import duties and import VAT always payable?
No. Customs duty is waived if the amount is less than £7.
Excise duty is not payable on gifts of tobacco and alcohol unless the quantitative limits set out in paragraph 2.5 are exceeded.
Import VAT is not payable on:
1. commercial consignments eg goods purchased over the internet with an intrinsic value not exceeding £18, but this does not include alcohol, tobacco products, perfume or toilet waters
2. gifts, excluding alcohol, tobacco, perfumes and toilet waters, with a value not exceeding £36 and which comply with the rules shown in paragraph 2.4
gifts of alcohol, tobacco, perfumes and toilet waters that are within the quantitative allowances detailed in paragraphs 2.5 and 2.6.
3. There are a number of other circumstances where relief from some or all customs charges may be available. If you think your goods may be eligible for a relief you should contact the National Advice Service for further information.
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So, if not marked 'Gift' the limit to get it import duty free is £18 (slightly more because if the import duty is less than £7 they'll waive it).
If marked 'Gift' the limit to get it import duty free is £36 (again, slightly more because if the import duty is less than £7 they'll waive it).
Also, to protect the UK Market, the rates added depend on what the item is, Clothes are something like an extra 10% on top of standard VAT, then you'll pay a handling charge of around £15 for the courier having to handle all the customs paperwork.
So DONT rely on writing GIFT on a package, it doesnt matter, it's all down to two things:
1. The value of the item
2. Luck
Linkage:
http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?_nfpb=true&_pageLabel=pageTravel_ShowContent&id=HMCE_CL_000014&propertyType=document#P45_3453
