Exams - level of difficulty
Just an observation from someone half way through modular ground school ...
I'd say the level of understanding required in the ATPL exams is close to or at A Level science standard - certainly in theory of flight, instruments and aircraft performance - also in parts of the aircraft systems papers - hydraulics, piston engines, gas turbines etc.
Other subjects like human performance are essentially a memory test, albeit interesting !. If you are numerate the mass and balance should be very straightforward. The two comms papers should be a walk over for any well trained UK PPL pilot.
Some of my friends on the course left school before 18 (or have been out of education for a long time) and have had some struggles with what may be seen as the "academic" parts of the course.
I know that Oxford are considering either a foundation course or an entrance test or both for future modular students. Seems a good idea & their pass rate is already high.
You need to take a good look at your own experience / competance so that you will get the most out of the course that you select - For myself I found that having read all Trevor Thom's books for PPL cover to cover was a very good preparation. The thing that you will not be prepared for is the sheer volume of information that the ATPL course will require you to absorb.
Your choice of distance learning or modular course not only depends on the depth of your pockets, but also on how long you want to take with the exams and how you rate your self discipline of getting down to the studying.
Hope this helps, best of luck,
D 129