"Of course, one of the best places to be would be somewhere like the Isle of Lewis, up in the north of Scotland. One of my friends lives there. Sparsely populated, close-knit community, little access to the mainland, etc.."
Yep! Islands are good. My family has a holiday home and relatives out in the Finnish Archipelago. It takes four ferries to get there from the mainland. If something like the rash ever happened, there would be food, water and fuel until the next summer. Old, wood-heated stoves would be brought back into use and most houses can be heated by the wood of the plentiful forest. Wells that can provide drinking water are commonplace. Vegetables and staple crops could be grown, fish caught and the locals would start planning and building for the years ahead. The island's hunting club and the ability of the locals to work together would be invaluable. But that's the positive scenario.
Sadly, I think if it was The Rash, we'd be relying on a harsh early winter to kill off beasts that can cross from island to island. It would be difficult to stop them otherwise. The other problem would be other people with holiday homes attempting to escape the spread of The Rash by boat but probably bringing it with them. Also, those with holiday homes would be reliant on locals to feed them after their tinned food ran out. I'd expect ugly conflict. Another problem would be a lack of essential supplies like bullets, spare parts for engines, oil, raw materials for making new products, etc. Hazardous trips to the troll-infested mainland would become a necessity.
A terrifying scenario.