I did CPL TK with CATS several years ago.
In theory there's a significant difference between the two, in reality they're pretty similar. We mostly practiced using ATPL practice papers, and the CAA papers I took at Gatwick weren't much difference to what I'd practiced on.
There are areas of the ATPL syllabus where they gave us the notes (and I still have them on the shelf - CATS notes were excellent then, and presumably are no less good now) but told us not to worry about it. It was maybe 10% of the total.
The biggest advantage was having to do a few less exams because of subjects being combined in CPL that were separate at ATPL.
A rather daft flaw in the whole system is that as I only did CPL, I didn't have to do IFR comms. But, holding a CPL, I was then exempt the TK exam for my IMCR [IR(R)]. Needless to say, like anybody else with any sense, I made sure I understood the material anyhow !
My appraisal of CATS (and I'm also a professional educator - specifically a qualified university lecturer, still in occasional practice) - superb notes, you're very much trusted however to sort your own study strategy, lots of helpful practice papers - although it's worth also subscribing to another commercial question bank: it's easy to get trapped into question spotting but they do try to get you to learn the material properly before going through lots of example questions. They are however extremely shambolic when it comes to organisation of the classroom periods, and everybody gets frustrated about that - although the actual teachers are generally pretty good with a lot of relevant professional experience. I'd be quite happy to recommend them again, with cautions.