The thing about pilot aptitude tests most have never been validated, so we really do not know if they work. Let us say we do a 1000 tests, how do we know the ten per cent we selected were the best.
In respect of aptitude tests with Oxford and CTC they are a joke as the main element is you ability to pay them a large sum of money.
Treat aptitude tests as a hoops you need to jump through, but my advice is as follows:
(a) Completely agree with jamesgrainge, an demonstrated interest in flying is paramount, either learning to fly gliders or powered aircraft, or working or volunteering at a flying club.
(b) A good grounding in physics, maths, meteorology and aerodynamics, the later read kermode.
(c) The interview is critical, you need to demonstrate interest and knowledge in aviation, by way of example. What are the four forces in flight? Tell me the the difference between a turbofan and turbo jet?
(d) I suspect most airlines feel they need to do aptitude tests to tick a box, but really it is mostly down to the interview and application. For example I once asked a prospective airline pilot, why he wanted to be a pilot, he stated that because he liked the cudos and wearing a uniform, wrong. Good answer would have been, I like the challenge of using a variety of skills, such as maths, physics, understanding of weather, working as part of a team, providing a good service to my companies customers.
(e) I really do not agree with companies who think they can 'improve' your aptitude. There are alot of wise and older pilots on pprune who would support you on here for free.