does the flat come with a shed?
what about seeing if there's a neighbour that you could perhaps rent an un used shed from. you never know there might be someone on your street not using one... leaflet drop perhaps explaining your problem?
I had the same problem for years. Renting commercial space is a no-no for a hobby, and sheds can be a dodgy option depending on location. Junkies and other no-marks see them as an easy target, rather than get a job and earn money. I currently work in a 16'x9' wooden garage, which is as well secured as I can manage - and in clear view of my sitting room window.
Unfortunately us renters are quite restricted. I moan about my 4x6 shed, but at least it's something so I can still do my hobby.
As long as it's secured well... if the area is dodgey, you should be ok.