Taxes
.... All the prices in London and the UK includes the British sales tax, VAT (Value Added Tax), of 17.5%. You can get a VAT return when leaving UK if you request the form at the shop. You'll need your passport for that. You'll present this form to customs at the airport and receive the refund in one of three ways: in cash right there, in the form of a credit to your credit card or a cheque by mail.

Shopping areas
.... London is a great place to shop. If you don't have too much time, just go to Oxford Street (Tube stations: Marble Arch, Bond Street, Oxford Circus, Tottenham Court Road) and spend your day there. You'll find some of the best shops in London, such as Gap, Dixons, Deberhans and Next. Go to nearby New Bond Street to designer shops like Guess and Calvin Klein. Also near Oxford Street are Regent Street with some nice fabric shops, Tottenham Court Road for electronics and computers, and Charing Cross for books. Don't miss a visit to Harrods (Tube: Knightsbridge) and Liberty (at Great Marlborough St from Regent St).

Clothing sizes
.... Suit and shirt sizes are the same as US sizes. For dresses, blouses, sweaters and coats, just add 2 to the US size and you'll get the UK size (US's 6 = UK's 8). For shoes you need to subtract 2 from the US size to get the UK size (US's 6 = UK's 4).

Bargain
.... If you don't have too much money or value what you have, forget about Oxford Street and go to places like Queensway, a great street for shopping. The indian-owned shops have much better prices and the staff is willing to bargain with you. Beware of perfumes sold at stalls on the streets (always with a very low price). They're either stolen goods or fake ones.

A word of caution
.... Beware of pick pockets on the streets, specially near cinemas and theatres. Also avoid to give money to people begging on the streets. I know you have a good heart, but these people are not what they look like. Most of them have a place to live and don't need assistance at all. Women carrying babies with a sad face are trained to fool you. According to the Police, most of the times the baby doesn't even belong to the woman...