Please bare in mind this was the process from over three years ago so it may have changed. However, there have not been too many cadet entries since...
Group exercise - Standard stuff.
Tests:
Verbal reasoning. If you have not done one before make sure you have a practice before you go. There are plenty to try online. They do take a bit of getting used to.
Maths. On our assessment day this was all aviation related, such as Glide slope checks, 1:60 rule, speed distance time etc etc. No calculator allowed. Cannot remember if you could use paper.
Technical questions: Standard ATPL theory.
Finally, there is a quick personality questionnaire. I hate these things as you cannot help but feel you are contradicting yourself. I think it was the one with 3/4 statements, and you have to mark which is most and least like you.
That’s all that I can remember unfortunately. Try to enjoy the day. They are a friendly bunch, and do want you to pass.
If you get through, (and it is the same process) the next stage is an interview with more technical questions, standard interview stuff and a presentation on something aviation related such as wind shear. If the interview goes well, then it’s into the sim for a quick check.