I think its the aviation authority that make the general standard of schools in a country. In the UK you know that the CAA are largely on top of their commercial schools, so while some are better than others, you won't really find a straight out bad one. Plus, the standard required to pass the flight tests will be high and consistent. Feels odd speaking up for the UK CAA, so to balance it out, they are still grossly overpriced and £84 to remove a restriction from an FI rating (one word removed from the license) takes them 3 weeks.