Okay, hear me out. I know the Erins said that it was originally in the New Forest, and then later was not based on any real place. I know. But I haved worked out where it most makes sense for the books to be set: Scotland.
So there are three main characteristics of where ever the books are set:
1. Positioned to the east of a sea (the sea in the books is to the west, as the sun sets in the west and this is the 'sundrownplace').
2. Has fairly large mountains.
3. Has characteristics of British countryside (in land features, and the twoleg maps have Britain-style road names and layout. Trust me, lived here all my life).
4. It snows, like properly thickly snows, in winter.
England has very little mountainous terrain, and certainly not near the west coast, so it's unlikely to be England. Also, snow in England is pathetic. Wales is more likely, but I still struggle to find places in Wales with mountains that close the coast and have twolegplaces where people live (which discounts Snowdonia) and have proper, really thick snow in the winter.
This leaves Scotland, which hits all this criteria, especially in the north. So there it is, my headcanon as to the setting of Warriors.