A certain online auction site can offer great deals. I'd recommend lots self seeders like foxgloves and ox eye daisies that perpetuate themselves over the years. And plants like celandine and snowdrops that amass and spread each year.
+1 for plants that like to self-seed etc, but be careful with foxgloves because they're supposed to be poisonous - otherwise I would have offered some of mine! I'm in South London and have about 100 self-seeded little plants at the moment.
Out of the Spring flowering bulbs (snowdrops, crocuses, native bluebells, tulips and daffodils), I've found crocuses to be the happiest to settle in multiply in a ~3 year timeframe. Bluebells will take over given half a chance but IME they take longer to settle in. I've found snowdrops to be fussy as we lost a lot that I planted last year (possibly because of poor drainage), but once they're happy they should spread easily.
It's not south east London more east London but I have had amazing bargains at Colombia road flower market.
I have stained my orange fences dark green and it has made such a difference.
Are you near an aldi or lidl .really cheap plants and garden supplies, morrisons and wilko are really good too
Check that the things you plant are not poisonous (like foxgloves ) and not prickly. .maybe avoid roses...