Read Thread: the state of letterboxing in England
I was there this past summer and looked up letterbox hybrids on geocaching.com. I was out in the country and with people who weren't going to drive far just for me to find a letterbox. It was the one way I found to easily locate possible letterboxes near where we were staying. It's not the solution for everyone, but it increased the possible boxes I could look for. I ended up finding two listed on Atlasquest and one geocache hybrid.