I think it is very low risk everywhere as it is so rare. But as it's horrific it's worth being vigilant - any sign of any lesions on the legs get straight to a vet. And yes wash off mud after walks. My mum's lab was one of the earliest Alabama Rot cases in the country - and the first dog to survive it. It's awful.
It's been so dry lately that I hcan't heard of many cases; however we often camp in New Forest and I do like to give the dogs a diluted Malacetic bath before bedtime ... I watch the ticks and grime of the day wash away and it makes me feel like I am doing something.