Your A-levels will give you all the academic knoweldge you need for the course. The main problems with the ATPLs is the sheer breadth of the syllabus. They don't go into great depth in most subjects but they require you to know a massive amount of stuff that is often not related. It's all that learning and re-learning that takes the time (by the way I've got a Masters Engineering degree from Cambridge and I found it hard!)