Yes, but if a 15A was available in the UK, would they have cut it? They still could have got the same audience as younger children would have still been able to get in.
Of course it would still have been cut. The BBFC deemed the uncut version to be unsuitable for 12-14 year olds. A 15A would've allowed any child up to 14 to see the film with an adult present, little different to the 12A. That the film was given a 15A uncut in the ROI shows what a bunch of prudes still run the BBFC. Any child can see the film uncut with an adult in the ROI.
The problem here is being created by Fox's desire to have children being able to see an adult action film in a country where it believes the adult cinema going public won't mind being treated like children.