purplemass.com

Try this site for listings.

http://www.boot-fairs.co.uk/London.htm

http://www.carbootjunction.com/search-by-town.php

http://www.countrysidepromotions.co.uk/

http://www.ibootsale.co.uk/

http://groups.msn.com/CarBootSales/_homepage.msnw?pgmarket=en-gb

These have pretty old info, but you never know:

http://www.car-boot-finder.com/

http://www.carbootcalendar.com/

And here are some examples in SW London:

Sedgehill Car Boot Sale – Catford
Sedgehill Field, Sedgehill Road, Catford, London, Greater London, SE6
Phone: 020-8309-5696
Map

Battersea Technology College Car Boot Sale – Battersea
Battersea Technology College, Battersea Park Road (A3205), Battersea, London, Greater London, SW11 5XX
Phone: 07941 383 588
Map

New Covent Garden Market Car Boot Sale – Lambeth
New Covent Garden Market, Nine Elms Lane, Lambeth, London, Greater London, SW8
Phone: 01895 639912
Map

Shepherds Bush Car Boot Sale – Sherpherds Bush
Silver Road, Aerial Way, Sherpherds Bush, London, Greater London, W12 0RP
Phone: 01753 663656
Map

Wimbledon Stadium Car Boot Sale – Wimbledon
Wimbledon Stadium, Plough Lane, Wimbledon, London, Greater London, SW17
Phone: 01932 355538 0870 840 8905 (Thanks to P Morgan for supplying the correct number)
Map