Thanks bjcc, like I say I make no assumption as to the current case... I am aware of the risk of upsetting the constable involved. However, crews are accountable to the company for, among other things, delays- in this hypothetical case, unless the PC happens to have a breath-test kit immediately available, the delay is likely to cost the company money. I suspect my employers would be less than happy with my conduct if I did not take all reasonable measures to eliminate that delay.
If the PC in this case had genuine reason to believe a test was necessary, I would be very curious to know what that reason was. I would be very unhappy to discover it was just a power-trip; I was brought up to have more faith in the British police than that.