Not sure if you can get them there in the UK, but here in North America, you can buy a Rubbbermaid or similiar brand tote box at most hardware stores and Walmart and stores like that. I bought my 20G tote box for $6, and although I only use it for aging water for my water changes, I know of a few people that use it for hospital tanks. They have many other sizes there, at reasonable prices...
A big advantage is that for the price of a 20G aquarium, you could buy a much larger tote box. But remember to take into account that you'll need more medicine the more water your Hospital/QT tank can hold...
Be careful if you decide to use a heater, as it might melt/disfigure the box. Use a HOB filter, and you're all set. Sure, you won't be able to see the fish, but for the price... Another disadvantage is that you can't cover the top or else it'll mean total darkness for the fish, and although I know that's good to reduce stress, I'm not sure what'll happen if you continue to house the fish there for long periods (more than a week, or even few days)
HTH.