Most light aircraft of foreign build are Metric, but the Lycomings and Continentals are all A/F and indeed use Whitworh as well on cylinder hold down studs etc where the sizes in the manual refer to the diameter of the stud / bolt and not the nut..
On the OCU we had Wessex and Puma, one metric and one A/F
The M1777 was not designed by BAe, it was a Vickers design that was developed on when BAe took over Vickers, whether they then swopped out the bolting system is the question, but as the USA was an Imperial user probably not.