Couple of cellphone towers on the property maybe, leasing some land to other farmers, and/or he did a bit more before his stroke and now just focuses on the sheep? All pretty plausible

Farmers in Wales are pretty canny and they have some unique advantages due to the hillyness. Leasing out land for mobile towers is one option, as someone said above, but I've also seen them group together to build small hydro power installs in their streams, or set up solar panels, and sell that power to the national grid. Sheep are pretty much all you can raise due to the geography of the place.

Wales is poor, outside the cities. You can still buy a house for £10k here (it'll be shit obviously). He'll likely get some subsidies and his pension now too, and I imagine he probably trades for some of his food. Eggs for fish, that sorta thing.

He's 70 years old. Nothing said his sheep were the only source of income he ever had over his entire life.

he eats for a few bucks a day, never eats out, never travels, etc. he probably doesn't need much cash to live