Scotland seems a strong possibility. They went there a lot to climb and wild camp in their younger days, as recounted in The Wild Silence, and married on Skye — and, if we’re to believe TWS, had actually always intended to buy land and settle in Scotland, not Wales. Wales seems to have been an accident when they hit bad traffic on the way to Scotland on holiday with small children, and turned off the road north.
In fact, weirdly, TWS, which I own but hadn’t read till last night, has the odd effect of making the SWCP walk of TSP sound less impressive and singular, because they seem to have hiked long distances in challenging terrain, and wild camped in way more edgy circumstances, when they were younger. You could have written the same walk as ‘joky account of how two creaky middle-aged people went back to the hobby of their youth’.