With a big firm such as GT or PKF, do most of the trainees go onto a ACA/ACCA/CA?
I think you should address this question to HR or training dept at these firms. Also take a look at the ICAEW website which is very comprehensive and shows you all the routes to training to be an ACA.
Similarly, look at the ACCA wbsite.
If you join a big firm and do AAT they tend to put you onto ACA/ACCA afterwards
I know someone with a top 20 firm who has just moved onto ACA