I draw a polygon, and strike a line through my user's geolocation. If it crosses the lines of the polygon an even number of times, it's outside. If it does so an odd number of times, it's inside.
The problem is UK counties are usually formed of natural borders like hills and rivers. They aren't tidy polygons with straight sides. That means arbitrary rectangles which fit the counties inside them will end up including a lot of area which is outside the county I am trying to target.
So, where can I find/generate a description of the county borders in lat, long points?