Is it possible for me to secure a role in product control or similar in a Top IB (eg. GS or JP) if I have not trained at a Big4?
Short answer - it is possible, but very difficult/ competitive. A large amount of qualified Big4 accountants consider this move as their experience is directly attributable, the hours are more reasonable and the pay/ experience is good. So you will be competing with these people. A lot of the Banks also only request Big4 hires through the recruiters.
My advice would to put yourself in the strongest possible position in terms of your experience and networks. Have you considered if additional qualifications would be helpful? If you are unsuccessful, you could consider a stepping stone role with a smaller or challenger Bank, and then use this experience after 12 months to transition in to one of the Top banks.
I hope this helps! Good luck.