GCSE is equivalent to A levels correct? In my country, A levels is considered upper secondary level, which is what they are looking for. I'm not sure if they will accept 5 C's or not, as normally they would want high grades in Maths and English normally. Have you tried sending them an email to ask? You can still make it for the selection for June/September intake. I was told that I have to go and do the assessment and interview before April.
Anyway for those wondering, I'm not 100% sure whether they give us the advanced compass or not, I did a basic compass test before and I do not remember it being this difficult.
What I did at STAA was :
-hand and feet coordination
-mathematics test
-orientation(i think it's called? it involves the ADF, compass and turn coordinator, just match the plane that is in the correct position in relative to the station)
-memory test (inputting new data into the autopilot while watching out for the warning lights, putting them out ASAP)
and a couple others that I can't really recall at the moment.