Hi Aiden, this doesn’t directly answer your questions, but I wanted to share a few thoughts as background on how the current CS cities were imported.
The big difficulty with bringing cities into CS is finding something that a) covers all the map, b) doesn’t have overlap with existing areas (this currently breaks street counts and challenges, see the discussion here), c) the areas being available as boundaries in OSM and the hierarchy being at least somewhat consistent so that it can be done in bulk.
For most European countries, including UK, a) means that rural areas will often be very large since there isn’t a consistent layer below (that also has a boundary) in OSM. The issue of overlaps should eventually get sorted out (I hope!), possibly in a way that would also allow for more manageable challenges than 10k streets. c) is probably the least problematic, since the process of importing entire countries is (mostly) completed. OSM data is messy and inconsistent, every country has its own historical quirks and oddities, finding a way to be consistent on a global level is hard.