mate if you find it hard to find a abandoned house (plus its a pain because of neighbours and stuff) try like planting around rail ways.

BUT DONT TRESPASS ON THE RAILS!!!

like sometimes they fence off a lot of land for rails with bushes, trees etc. etc.

you could probably find a place there (depends on how much into london you are) or just find a field thats not used for much like next to some industrial places they sometimes have fields around. again helps if you live somwhere betwean zone 3-6 but if you want to only come back in summer than you could take a tube to the places like 'dagenham east' or if west is your favourite than try 'uxbridge' them places should have all sorts of fields and stuff everywhere.

Also try to come see your plants at least a few times during the summer :P

Lol not that it will be easy to stay away, trust me

You start to miss em after a while :P

Good luck