The OSM wiki has a list of UK Mapping Priority areas, where there are a presumed substantial number of missing roads which need to be mapped. These can then get mapped at a mapping party. The probable introduction of a freely-derivable “complete” set of Ordnance Survey road data in ten days time may mitigate the need to do such first-step mapping, but for now there are many areas in the UK which could do with some on-the-ground data gathering.
As an alternative way of identifying such areas, I developed the “Lonely Buses” map. As an extension to this, I’ve now created a “top 50” list of areas that have the most number of bus-stops away from roads, i.e. areas with probable missing roads. You can see this list here. A basic form of reverse geo-coding is used to assign a name to each tile.
The list is automatically updated weekly. It is flawed in that it considers each “tile” exclusively, so if a town is split in half by tile edges, it will appear twice and lower down the list, compared to a similarly incomplete town being in the middle of its respective tile.
Looks like the Newcastle area is the No. 1 target area (within areas which have the government bus-stop dataset imported.)
Andover has a mapping party planned for mid-May and is currently No. 39 on the list. If you are in the area, sign up!