Yes, I had to make a change about urls in order to support locations from all around the world. lat/long ended up being the only unique thing in common.
When I updated the way meet.hn handles location, I migrated all city ids like the one you mention "gb-London" to a new one using lat/long.
The problem is: when users select a location, they can end up at another (close) location suggested by OpenStreetMap. To mitigate this pb, if locations have the same full name or OpenStreetMap id, I don't display them in the location selector.
Also, I put locations of type "city" first to nudge users into selecting cities more and hopefully end up with less problems of this kind.
I made a (now deleted) root comment about this, but saw someone else commented about it to.
All the links people have in their profiles basically doesn't work anymore. Maybe you could at least setup a redirect from the URL without the coordinates to the one with the coordinates, assuming when they set up the link, they used whatever first result you got back then. Basically run both versions (not literally), but redirect from the old one.
Good practice to keep URLs alive one way or another for as long as you can, as effectively all the links the initial group of users put no longer works as expected.
To do so, I implemented a search using OpenStreetMap. The thing is, OpenStreetMap can give several results for a given location (and rightfully so). Example with Paris: https://www.openstreetmap.org/search?query=Paris%2C%20France.
When I updated the way meet.hn handles location, I migrated all city ids like the one you mention "gb-London" to a new one using lat/long.
The problem is: when users select a location, they can end up at another (close) location suggested by OpenStreetMap. To mitigate this pb, if locations have the same full name or OpenStreetMap id, I don't display them in the location selector.
Also, I put locations of type "city" first to nudge users into selecting cities more and hopefully end up with less problems of this kind.
Code about this is here: https://github.com/borisghidaglia/meet-hn/blob/808143d36841b...
There is something to be done, but I only had overly complicated ideas to fix this.
Very open to suggestions. Don't hesitate to open an issue/PR for this on GitHub.