On Saturday I noticed that every second shop in East Dulwich seemed to be selling trees outside. You would imagine that with this level of competition, prices would be competitive (though actually that seems unlikely if I think this through).
The Sainsbury's at Lower Sydenham had trees last week - a larger one is ?25 - they are probably 5 ft, not enormous. The Perry Hill pub (on Perry Hill...) also had them at the weekend - same price. I think smaller ones in both places were ?15, but could be wrong.
I got mine from the Perry Hill. It looks quite nice, though has dropped enough needles to give me a new green carpet in the lounge - still has plenty of green on it though (well it did last night). I think the guy outside the Perry Hill is doing free delivery, though I carried mine in the car, so not sure.