The answer seems to be either 64 (if we were to create a group for each city) or 184 (one for each county).
Even if we choose cities:
- most of them would probably have no members at all, and
- probably half of our visitors would consider their nearest city to be too far away to be of interest
- we no longer ask even what country members are in, but based on the old site, I’d guess that more than half of them are not in the UK.