I know this thread hasnt been active in over 10 years, but I thought I'd add my ideas incase anyone was looking for the same info I was looking for.
My favourite idea is getting a white safety fence with a gate. The ones I've seen are about 60cm tall and come with 5 pieces, which would be enough for the two sides of my double bed that aren't against a wall (I think that would be about £50), though you could buy a second set to cover 2 more sides. I'm not sure what the best way to keep it in place would be and it would be expensive, but it's probably cheaper than an abdl crib and solves the issue of people possibly seeing it and portability.
My second favourite idea is getting a play pen, likely one where you can order any amount of single pieces as you want. Its about 40cm tall, it would be £100+, but it would look cute, it wouldn't be cold like the fence (could be good or bad depending on if you like the idea of temperature play) and you can have some interactive pieces. There's the same issue of finding a way to keep it in place as well.
My least favourite, but most affordable idea is Eva foam mats interlinked around the bed. It would take quite a few to get a decent hight, it won't be very sturdy and unless you cut them there arnt any bars. But, to go around all sides of my bed with a hight of 20cm (past my mattress, it would take 2 lines of them) would cost about £30. Since they would be going around my whole bed, pinning some in-between my bed and the wall should help keep the ones around the outside sides up, but if that doesn't work, or if your bed isn't against a wall lining the outside of your bed, underneath the mattress and then linking the others to those at a 90° angle should work, though you'll need double the amount. it's a great starting point for anyone who wants to dip their toes into abdl furniture in a cheap and inconspicuous way. Plus if you get one with clean edges you could leave some up as a makeshift headboard.