It is worth discovering 'Roots and Shoots'. Just outside SE1, on Walnut Tree Walk SE11, on your left as you enter from Kennington Road.
Its a long standing project that involves training teenagers with learning difficulties in gardening and carpentry, with providing education facilities for local schools and people. Plus they sell good quality plants. The garden is open to members and has a great newt pond, a beehive, interesting planting including a meadow and nettle bed, and some curious sculptures. They also have regular open days where disparate parts of the 'community' pull together. Last time there was a 30 strong choral group singing. You also get good advice when you buy as people know their plants. All sounds a bit worthy, but it really is great, and kids love it.
The Museum of Garden History by Lambeth Bridge also has regular plant sales. And a good cafe.
For cheapness try the flower market on Nine Elms Lane. They have some really big things in pots and trays of potting plants. But it closes around 7.00am!
For specific plants - what does grow in a north facing lightwell - I sometimes go down to Wisley gardens in Surrey, the RHS place. There is a huge choice, everything is top quality, lots of advice and you get a day out into the bargain.
There' a smallish garden centre up Camberwell rd. Quite reasonably priced and a nice selection. It's past the centre of town and up the hill a ways - get off the bus just after the hospital. Sorry, cant remember the name of it, it may be the grove nursery.
The best one I've been to in the whole of Southwark is the Dulwich Gardening Centre. It is right near the train station on Dog Kennel Hill, right down the hill past the big Sainsbury's. There is plenty of space to park up the side aswell. Their plants are fantastic, although their containers are well expensive. We get our plants from Dulwich and our containers from B&Q in the Old Kent Road.
There is an excellent garden centre in crystal palace called "the secret garden". Its near the safeway there. V extensive range of plants and better priced than Dulwich Garden centre (in east dulwich). Theyre v knowledeable and will send you out a regular news letter too.
Also try columbia rd on sunday mornings - but this is best for bedding plants at knock down prices!
The plants at b&Q are v badly looked after - as is all the stock is their terrible store.