If you're strapped for space, you could also try growing gooseberry & blackcurrant bushes in large containers somewhere in your garden, then when you eventually get your allotment you can transfer them there. Do you have a front garden? if so, you can always do what I've done and instead of planting shrubs or flowers, I've planted soft fruit bushes in the borders. Not a very traditional use of space I know, but as front gardens as otherwise often wasted, it has resolved my problem of not having enough space for them in my back garden.
Aubergines will also grow in your mini greenhouse. They like a slightly warmer climate than UK outdoors. Chillis will also do fine in there.
Swiss Chard (rather like spinach) now also comes in a variety called Bright Lights, and has very decorative yellow and red stems & leaves so a few plants can be dotted in your flowers borders. You will find the seeds in a good garden centre but you can certainly start them off in trays in your mini greenhouse.