Trouble is, where do you join the lines? For example being in Essex would mean I'd happily pop to Herts or Suffolk for a meet, so maybe you'd tag those together. However on a nice day or for the right meet I'd happily go to Sussex or Surrey or Norfolk, so you'd have to include those too. But then people in Norfolk might not travel as far as Sussex full stop, so it's pointless including those in a section together: Grouping only works for the individual, not en-masse.
The only way to do it is to encourage people to subscriber to each area they're interested in, and just accept they're going to miss a few things. Can you even subscribe per sub-forum on here? I don't know.
*EDIT* Yes you can, I just checked.