Sending or receiving mail in a foreign country can be a bit trying, at least until you figure out the system. To minimize hassles, get assistance from London's main post office:

24 William IV St
Trafalgar Square
WC2N 4DL
020-7484-9307, or 020-7930-9580
Mon-Sat 8am-6:30pm
Tube: Charing Cross and Leicester Square

Unlike the main branch, most post offices are usually open Mon-Fri 9am-5:30pm and Sat 9am-12:30pm. Mail can be deposited in any of the red Royal Mail boxes, and stamps are often available at newsstands.