Hi Tracey, each market within the london area has it's own individual customer base...some markets are good for one thing, and not for another...you really need to research before rushing head first.
I to am doing something similar to what you seem to be selling but my lines are more vintage...I've been to most markets in london doing research as i'm now on my way to setting up a stall for 2-3 days a week, so will only be operating as a casual.
Some markets won't let you sell as a casual, and only full time, others are open to casuals but will only offer a select amount of spaces which fill up quickly on the day, so you need to be up extra early and waiting in line to be appointed a space. Other markets in london won't let you sell specific clothing/goods if it clashes with their other longer term traders, and this is known as a clashing policy. They could let you sell but will give you a random spot which might be off the beat and track. Other markets are specialist, so would want to know what your selling, and if to close to what you find on the high street won't allow it, as it doesn't add to the uniqeness of their market. This usually applies in the more tourist areas of london like portobello, Camden etc. I'ts very hit and miss, and i would advise checking markets on the national market traders federation, and spend a good week or 2 visiting each 1 and making notes, about competition, customers, area, speaking to management, checking rates etc. I have researched alot of markets so if you have any questions feel free to fire away.
Were you looking to sell as a casual first or looking to do full time? Different rates apply at markets, and you will get better rates as a long term trader. Since your from Herts, i would advise checking out St Albans Market, on the high street. This operates every wed/sat. Prices are approx £40.00 for the day, more on saturdays and it has a good passing trade, since its a very well established market. I'm considering selling there... however, they operate a clashing policy, so maybe have a look at competition before getting to involved. Secondly watford indoor market is good, and has a good footfall, but they don't allow casual trading. You invest in the stall and design it accordingly with the intention of being there long term, and i think it's a deposit upfront of 3 weeks....they offer good rates and half the normal rates if you sell anything not found anywhere else in the market.
There are quite a few markets in the Hertforshire area...Wendy fairs who run wembley market also operate Bovingdon market, and this is huge...But only operates on a saturday i think, and slightly to far for me, but may be ideal for you. Some people can make a weeks wages just on the 1 day there....
Good luck with your search.