The flags on the Ariane launcher are the shareholders of Arianespace Launch Services, and the UK isn't one of them. No idea why not.
If I remember correctly, membership isn't just about paying money. Each country benefits from their contribution in the form of contracts with businesses in that country, so the UK must be selling something or provide some service to ESA. There's also a division of ESA opening (or is already in operation) in the UK.
ESA is already participating in a planned manned mission to Mars (Mars 500), conducted by the Russian space agency.
http://www.esa.int/SPECIALS/Mars500/
I don't think it's only about the money that the UK doesn't have its own space program. No nation can go to space alone (not even the mighty USA). To undertake something like a manned mission to Mars or a moonbase, the world needs to work together.
ETA: undertake: classic case of Dunglish. 