Best way to send a gift card from US to UK ?

I would like to send a quick gift card to my friend in UK for her birthday. I need suggestions for buying a gift card online - it can be a store card or even better a visa/master card which can be used at more places.. I do not know the stores in UK (except for the expensive Harrods). What is the best way/site to buy a card which can be shipped to UK or used online.. Pls suggest a decent store or a generic card (visa, etc) and how to. Many advance thanks !