I've got several modern non-US machine guns that I will never use. Problem is I do not know off-hand what British machine gun is used on the Challenger.
If this is the Tamiya Challenger I (Mk 3), I have that kit and can compare the turret machine gun with one of non-US machine guns I have on hand. Also the machine guns I have in the modern weapons sets tend to be dismounted infantry types and not the turret mounted variations.
The other posters are steering you towards US style machine guns which would not be appropriate for a modern British tank.