Yeah easy to say "get in your garden" but I don't know anyone who has one to get in.
Allotments seem to be big here.... although you can't really call them allotments, as I suppose that implies you were allotted it by someone or something; as far as I can tell if there is an empty bit of land then people just put crappy fences made of old bits of plastic around it and take it. Kinda like a claim in the old west. Once they've got the bit of the land they tend to strew it with rubbish, leave a dog tied up howling mournfully all day and all night... and possibly plant a couple of potatoes.
Sometimes they build houses, there is a bit on the edge of Lisbon which is a kind of illegal semi-shanty town.
i was at an adopted site like that in valencia, which a collective had nicified and ended up fighting the municipia who of course realised they wanted it after all
i somehow lucked out with the only allotment i ever heard of in london without a waiting list, opposite my home. it's much as you describe though, mostly overgrown with brambles and bush, plastic flapping gaily in the breeze, rarely anyone about, not much veg at all, dilapidated plots and derelict outbuildings, the whole site neglected and desolate
thank goodness, communal horticulture is the way to get at the space, but a properly run site would be less 🔥