Hi,
The Sydenham Community Garden (http://www.sydenhamgarden.org.uk/)has a couple of open day type events every year where they sell nice, cheap plants. I think every Tuesday they sell plants from the Sydenham Green doctors' surgery on Holmshaw Close as well, although it's always during the day so I've never been.
The Kent House Road allotments also have an open day where there are loads of plants for sale very cheaply, but you've missed that one for this year. There's also a plant sale in the Horniman Gardens at least once a year which is good but it gets very busy.
There are in fact two Open Days at our site each year. The one in May you know about - and thanks for the mention - but NSPCC Bromley also have a late summer one each year. This year it is Aug 31, 11.00 - 15.00. It will include a fruit and veg show and many of the same sort of attractions as our May event. There will also be some plant sales - probably including the 100 odd thyme seedlings I have growing in my greenhouse right now.