Finished my ATPL exams with Bristol Groundschool last week, I'm very happy with my decision on going there.
Half of my classes were made up of military pilots who use them based on reputation, and due to the most (if not all) Bristol instructors are ex-military.
They have many instructors, and each one is a master of their subject. The questions we asked in class should have been testing for most, but they were still able to give the answers in a way we could understand.
The cost between CATS + BGS Online subscription vs BGS is minimal when you consider the overall cost of flight training.
And as mentioned before, with the Wings Alliance up and running it might be best keeping your training together to better your chances of getting that elusive first job.
Let's not forget it was the school of choice for Prince William a few years ago.
Good luck with your decision.