The main shopping areas are on Oxford Street and Regent Street. There are now supermarkets in London including the Safeway and Sainsbury chains. There are also small markets in every neighborhood and, of course, the food halls at Harrods and at Fortnum and Mason. Be aware that many drugs sold over the counter in other countries can only be obtained with a doctor's prescription in Britain. If you do take medications, bring an ample supply with you.
Many phones now accept credit cards as well.
Mercury phones also accept Mercury phone cards and regular credit cards. Keep in mind that the phone system in London has its own unique sounds. When you pick up the receiver, you should hear a continuous dial tone. After dialing a number, you will hear a ring, ring, pause to indicate it is ringing at the other end. If you hear a series of rapid beeps, this is a busy signal while a continuous sound means the number is unobtainable.