Are you on 9.5? What you can do is create a new map for London and in Network Atlas in the group by drop down change to Site Location and drag those nodes for London onto a new map and arrange them as you desire. You can then associate or drag that new map onto the world map and user can drill down to that from there.
Is that the only solution? It won't automatically update when I give more nodes the 'London' custom property, and it'll mean I'll need to duplicate two maps for each site, one for network devices, and then one for servers (which then sits as an object on the network devices map).
There's no way to create an object which aggregates the status of all the nodes with a particular property?