The rules in England and Wales at present are that you have to get married in an
approved location. However there are plans afoot to licence the celebrant rather than the location. We don't believe that this is law as yet. You could check with your local Register Office or Council to see what local venues are approved if you haven't already done so.
The situation in Scotland is slightly different. See
here - you can get married outdoors but you will have to check on residency requirments.
Hope this helps.
Liz