I spent some months in London, twelve years ago. I attended every car boot sale I could locate during this time. There was a magazine that listed every flea market in England & Wales, it was sold at the entrance of some of them:

I used to go to three flea markets each saturday and sunday -providing I had no car, it was difficult to reach all of them before closing, just travelling by tube and bus. I used to leave home around 6.00 am and I wouldn't return until 4.00 pm xD I had to plan every move in advance, taking into account which underground lines where closed, so I could attend the more of them:

There should be something similar to this magazine nowadays, an online listing I guess, but I am not able to find it.
Besides, I wrote chronicles of each weekend (at least, those weekeds I could afford the car boot sale-raid) in the Amstrad CPC spanish forum,
here.
...and twelve years ago, it was worth to pay a visit to the Computer Game Exchange at Notting Hill, which was not so expensive. Also by that time, the most "profitable" car boot was the one around Brick Lane (particularly, the not-so-legal stalls located in Chesire Street, heading to Bethnal Green, and outside Shoreditch High Street Station). From that notes, it was also very "interesting" the car boot sale at Kilburn (plenty of software and hardware, I bought some Amstrad NC100/200, a BBC Model B, a GX4000, some Amiga games), and the one at Seven Sisters (Atari ST software), and Rotherhithe (this was little crowded, and I found tonnes of C64 games there).
Five years ago I could spent another two months in the Wallington/Sutton area, and I attended all the flea markets and fairs I could, again. This time there were less interesting things, and I can only recommend the flea market hold in a school at Sutton (large number of stalls, it is impossible not to find something there). At that time I spent a lot of money at Game Focus in Goodge Street. Don't know if it's still running, but I managed to buy some games there despite its prices very similar to ebay's
