Just done Bristol and they are an awesome group of instructors who really know their stuff and will get you ready for your exams. Several RAF guys do Bristol, also lots of guys who already have ATPLs from other countries. Bristol also provides great support. Can't recommend them enough. I never did CATS. I went with Bristol because of reputation and I liked finishing in only two exam sittings since I was doing an ICAO conversion and wanted to finish as quickly as possible.