North Maidstone was my choice when I was thinking about doing this myself.
You would capture the maidstone/medway population which i think is the highest in the county. 20 mins away is Ashford with the second.
Nearest competitors would be:
Scout centre (Maidstone) - No threat as the opening hours are allegedly bizarre and subject to change. Also quite small.
Karma/chaos or whatever its now called (T wells) - Never been myself but owner of a hold manufacturer described it as world class.
Arethusa (Rochester) - Not hard to offer a better venue than them as its dated and as part of a charity centre its unlikely they will invest or expand in size.
The reach (Woolwich) - Where i used to go before I built my own moonboard and indoor boards at home. Would be my first choice if i wanted to go to a wall.
Revolution (Ramsgate) - Think this place is small?
The huge folkestone centre under construction - Might be the biggest place in the south east when built? However based on the initial plans i have seen, all the bouldering looks too vertical to attract repeat customers. I don't think there has been any consultation with climbers as its also a skateboard centre, so might not be a threat. Plus folkestone is an arse to get to.