Daysleeper
Standard Noises answer is missleading.
Police now have the power to breath test anyone who falls under the R&TSA, that is ATCO, Pilot, Cabin Crew and Licenced Engineer, if they are invloved in an accident. (there are other reasons, which are covered to death on other threads)
Assuming that the driver of the tug was not a Licienced engineer, irrepsective any any desire to breath test him, they can't, there is no power to. Assuming that there will now be a triade of 'Well, there should be',, I agree.
Daysleeper
There is NO issue as regards to breath testing on a flight deck, if that did indeed happen. The advice you mention is an agreement between ACPO and the CAA. It's exisitance is not a defence, nor is it enforcable in any way, save possibly as a discipline charge against the an officer, and that would be stretching it. It also does not say that a breath test shouldn't be required and undertaken on a flight deck. It advises that any test should be conducted out of view of the public.