That's a good link from Zacsmaster. This map might also help:
http://www.streetsensation.co.uk/markets.htm
as might Wikipedia:
http://en.wikipedia.org/wiki/London_markets
Romford might not exactly be a tourist destination but it's got wonderful indoor and outdoor markets. (So much so, that coach companies throughout East Anglia run tours there).
The outdoor market has 270 stalls and trades on Wednesdays, Fridays and Saturdays.
http://www.havering.gov.uk/index.aspx?articlei d=386
I'm not sure about exactly what's going on at the moment with the indoor market (known as Romford Shopping Hall) because it was scheduled for rebuilding and I've not visited recently to see how this has affected traders:
http://www.landmanagementltd.co.uk/romford.htm
Rochester's a lovely town for a day out. There are some great shops but with hardly a familiar High Street name in sight. (They're all down the road in Chatham, Strood and Gillingham). There's also a good street market on Saturdays.
Hemel Hempstead is a surprisingly pleasant town. There's a great general market on Thursdays, Fridays and Saturdays. (If you're ever able to get there on a Wednesday, you'll find loads of bric-a-brac and antiques). Just outside the modern town centre, you'll find the Old Town. It's well worth a visit to see what the independent traders have to offer, as well as to find some great pubs and restaurants.
Chris