Don't spend £250/year. Spend as little as possible, then scale as you grow. Two options to consider:
1. Use a source code hosting service such as github
Github is a popular hosting service for the git version control system. In addition to code hosting and online code browsing, the long list of features it offers includes team management, wikis, forums, and bug tracking.
Github is free for open-source projects, and you can even host project pages and entire websites with it. Since good open-source projects use version control anyway, I think it's worth taking the time to learn git just to take advantage of github.
You may also wish to consider other code hosting services, such as Bitbucket, which uses the Mercurial version control system.
The great thing about services like these is that they encourage a community of active users to spring up around your project, because you've chosen to use a dedicated code hosting service that many are already familiar with and using in other projects they contribute to. Contributors can push fixes and new code to you, and you can review it and accept it into the official project. If I was starting an open-source project, github is what I'd use to promote and manage it.
2. Launch on cheap shared hosting
If you don't want to use a source code hosting service for some reason, I'd suggest shared hosting.
This allows you to host multiple domains and gives you an 'unlimited' amount of disk space and bandwidth. (If you read the fine print it turns out that 'unlimited' actually means limited, but only if you abuse their terms of service by, for example, hosting illegal downloads or large files that become very popular.)
Finding U.K. hosting companies who compete with U.S. shared hosting services on features and price is very difficult, unfortunately, and they are often many times more expensive. For this reason I tend to host my projects with U.S. companies now, even though I'd like to support British ones. I can recommend both 34sp.com and OpenmindHosting, but their basic packages support a limited number of domains (one for 34sp, two for Openmind) and their bandwidth/storage features aren't as generous, so you may find yourself paying for add-on domains and bandwidth faster than you would with a U.S. service.
Some people will tell you that you should host in the U.K. if most of your users are in the U.K., but in practise many connections are so fast now that hosting in the U.S. makes little to no difference to the perceived connection speed here in the U.K.