Only just found this - sounds a wonderful challenge and I am totally up for it. Plus I can knock off A and B within 2 minutes of leaving my house - I live on the edge of a town beginning with A, which directly abuts a village beginning with B, and both have signs next to each other on the main road that's no more than 400 yards from my door!
Looking through the Wikipedia page for my county, we're only missing a Y and a Z, although strangely V is the next hardest to get (just one hamlet, so not even sure it would have a sign - some don't round here if they're only ten or so houses and this is smaller than that). Most of the other letters I would have thought tricky, do seem to have multiple options available, which makes me optimistic.