If you're getting married in Kent, you can have a "Kentish Wedding Ceremony" and there are outdoor buildings like rose arbours that are licensed for marriages. It's the closest to an outdoor wedding you can get in England until 2005 when they revamp things.
In the US, the person performing the ceremony is licensed, and that's why you can get married almost anywhere. Heck - I'M licensed to perform marriages in the State of Florida! But in the UK, it's the place that's licensed, not the person. That's changing in 2005.
www.hitched.co.uk has a great venues list, with reviews and everything. Also has a lot of other wonderful tips and links - as well as a great forum for getting other people's input. Keep in mind that some of the registry offices in this country are in such wonderful, historic buildings that they're almost as nice as a church for aethetic purposes.
Good luck! Need any more help, just ask - just got married in March and did TONNES of research!