I feel that if you really know what you want to do in your life like Engineering or prehaps IT, then I would advise you to go to Poly because you can specialise straight away and if you even bother to put in the effort, you can make it to Uni easily coz most people don't really bother to study in Poly.
Poly is really easy compared to A lvls. Been there done that, I know.
However, if you got no idea with what you want to do with your life now, I recommend you go to JC because you still got lots of time to think about it.
Even after you finish your JC, you still can spend 2yrs in Army thinking about it too. If you go to Poly, that certain course you take will be probably what you gonna do for the rest of your life and at 16, sometimes you are not too sure yet.
You can study triple science in JC and still go do Accountancy in Uni if you want to.
A level is really not easy, be prepared to spend lots of time studying and feel the stress. But its the fastest, most versatile and most direct route to Uni.
I am a Jc student so take what I say with a Jc pinch of salt.
Anyway, one of the best thing is that Jc people ORD faster... My Poly fren enlisting next week but I Ord in 3wks time...
