To be honest, we don't really know what the buildings at Wroxeter looked like, it's guesswork based on the surviving groundplan, but it certainly doesn't look wrong. Maybe, it looks 'possible'. And I think 'watching some Roman villas' is exactly what the people who built the complex at Wroxeter would have done. There would have plenty of examples of Roman architecture around when they built it, even if there were no stonemasons or tile-makers left.
I'm not sure what the Wroxter buildings were roofed with, but Wroxeter is close to Wales where the best British slate comes from, so slate is certainly possible in that area.