green house?

If you check out Argos you can get one for less than £20, they are ideal for small gardens but not much good if it is windy, I bought one a few years ago and sadly it got destroyed, if you have a sheltered area it is ideal.

I should think you can just get the small plastic shelters for that price. They have limited use but are better than nothing.
Be careful if you are tempted by an offer in the newspaper of someone offering an unwanted greenhouse for free as long as you dismantle it and take it away. Replacement sheets of glass are very expensive and will rapidly exceed your £20.
Do you actually need a greenhouse? At the moment our local Lidl has a coldframe for £15 which would probably suit you. I am sure that there will be a Lidl in Basingstoke.

of cource there is

B & Q have a two tier mini greenhouse for £18.98 it isn't very big depends what you want to use it for really. Oh and Homebase has a slightly larger version for £16.95!

Be careful with the small ones, they can heat up extremely quickly as they don't have vents in anywhere esp. at the top, meaning that seedlings and plants can fry quickly!

I have a plastic greenhouse and have punched lots of holes in the front apex using a paper hole punch, for ventilation. It's a bit tricky to do but works a treat!