Overpass API: how to get all the values from a specific key

There's a few ways to do this, but behind the scenes they do essentially the same job. Note 'node' won't return all shops so I used 'nwr' (nodes/ways/relations) I use:

rel[boundary][name="Greater London"];
map_to_area;
nwr(area)[shop=car];
out meta center;

shop on its own returns 20mb of data so I restricted it to car shops in this example.

If you want to display the boundary use this:

rel[boundary][name="Greater London"]->.LDN;
.LDN map_to_area;
nwr(area)[shop=car];
out meta center;
way(r.LDN);
out geom;