You're unlikely to find any this early in the year - they are usually not available until late spring and summer. I found some margies in Brixton in early June last year, and others were able to find snails (margies and tigers) in Brixton and Dalston a month or two later.
If you want to try in a month or so, last year I found margies in the Market Row arcade. Come out of Brixton station, turn left, and then left again onto Electric Avenue. Walk down the street past all the stalls, and you'll come to the turning onto Atlantic Road. Turn RIGHT here, and stay on the right-hand side of the road. You will come to Market Row, a covered arcade/market - it has Market Row written on the front of the building. I think it's next to a bakery.
Go in and walk straight through til you're almost at the end. The shop I found the snails in is called (I think) WA-ZO-BIA or something very similar to that. It has crates of vegetables and fruit outside it. It's opposite a tiny store called something like "Electronics Warehouse", and a fancy-looking organic food-type cafe called "Wild Caper".
The snails were in a bucket near the counter, next to all the dried fish heads and other stinky stuff! The guys there are actually really friendly and helpful and they didn't care at all that I wanted snails for pets, in fact they helped me search through the bucket for live snails. Take some handwipes with you for once you get out of the shop, as you'll need them after digging through the bucket - mud and slime and snail poo everywhere.
Last year they were charging £3 per snail and they were BIG - make sure you have plenty of tank space for them. They're tough creatures but don't expect them all to survive no matter how well you care for them - I only lost one snail out of the six I picked up, but it was still gutting to spend so much time and effort on trying to nurse her back to health only for her to die a couple of weeks later. Thankfully the others are now very happy and healthy and have provided me with two batches of beautiful babies
You'll need to give them a very thorough warm bath when you get them home, as they'll be absolutely filthy and their shells will be caked in mud and dirt, and they'll probably be quite retracted or sealed in and the warm water will encourage them out. Put them in a very warm pre-prepared tank next to some food, and hopefully they will come out for something to eat