At this stage you need a general understanding of the breadth of how aircraft work for your interviews - I'd still recommend including a PPL book within your reading. Yes, these books are simplistic - but that's not necessarily a bad thing; it's very easy to become a bit of a snob about levels of information - a really good understanding of the basic level information will be of more use to you now than a shallow understanding of more advanced subjects that they'll be teaching you later anyhow.
Imperial, BTW, are very heavily into the mathematical aspects of engineering - so you do want to be 100% on your A-level maths syllabus when you go for interview.
Incidentally, I became a chartered mechanical engineer on the strength of an Aero-Eng degree, and I've known a few people who have become chartered aeronautical using mechanical degrees - yes they are different, but I wouldn't get too hung up on the differences and their career relevance - go for the one that excites you most. Either will be incredibly hard work for 3 or 4 years, and either will be extremely rewarding in both the short and long term.
Best of luck,
G