Here you go...
Straight from Hmrc site
VAT refunds for international travellers leaving the UK
If you are leaving the EU, the information below will help you understand if you may be able to get your VAT back on personal exports of goods.
If you live outside the EU
If you are an overseas visitor, to qualify for a VAT refund you must:
live outside the EU
leave the UK for a destination outside the EU with the goods you have bought by the end of the third month after the month when you bought them
show customs officials the goods, your receipts for them and a completed VAT refund document when you leave the EU
When you purchase the goods
First, you need to choose a shop that operates the VAT Retail Export Scheme. Look for a Tax Free Shopping sign. It's a voluntary scheme and not all shops operate it, so you need to check before you buy anything.
To obtain your VAT refund, you need one of the following documents. The shop will give you one of these:
a VAT 407 form
a shop or refund company's own version of form VAT 407
a VAT Retail Export Scheme sales invoice
You need to fill in the form when you make your purchases, in front of the retailer. The retailer will ask to see evidence that you are eligible to use the scheme, such as your passport.
You also need to agree with the retailer how your refund will be paid. Some retailers will pay you the refund directly, others will operate through a refund company, and some will have an arrangement with a refund booth at the point where you leave the UK.
You may not get all the VAT back. The retailer and/or the refund company may make a charge to cover the cost of handling your form. If they do, this will be deducted from your refund before you receive it.
When you leave the country
If you're travelling outside the EU, you must show your goods, receipts and your refund form to UK customs staff at the airport you're leaving from. Make sure you arrive at the airport early so that you have plenty of time to deal with the customs staff before your departure.
Hope that helps...enjoy your stay.
Surprised Harrods don't have a Festool concession in store!
Scott