We all love a bargain, and it seems almost any luxury/electrical/electronic item is available on here, really cheap, from China or Hong Kong in particular. Aside from the problems of a warranty being honoured locally for something bought far away, and the fact you may be buying - make that probably are buying - poor quality fakes of the original, or damaged/returned/re-graded stock, please be aware that even if it's the real deal, the price you paid for it on-line may not be the full cost to you by the time you hold it in your hands.
When the package comes into this country, Her Majesty's Customs & Excise officers have the power to open the item and inspect it, and whilst they are doing so, they may decide that from the price/value (which your sender has written on the Customs docket on the package) that they are very unhappy for this item to enter the country without Our Lovely Chancellor Mr Brown at No 11 getting several more pounds out of you. This has happened to some friends of mine, on four instances they care to admit. Two of them bought digital cameras, and two bought watches, and believed they were making a big saving on the UK retail price. When Customs were finished adding their 'cut', they all ended up paying more than they would have over-the-counter in the high street.
The Customs 'formula' is also highly erratic. It involves the price of the item, the postage and insurance charge, and any other number they care to factor in. The same item may have £10 added to it one day, and £45 added the next. There is no arguing with them. No appeal procedure. The item 'arrives', but you don't get it, not until the duties are paid, and you sign a form... Just what you want to put a sour taste in your mouth over the deal.
When buying, if the source is overseas, check that the item will be posted from within the UK. A hefty surcharge is not guaranteed, but it is Sod's Law. I'll not risk it.