The power to request an EVIDENTIAL sample of breath is conferred by Section 7 of the Road Traffic Act 1988. This section confirms that the device used must be type approved and that the request (for evidential breath samples) can only be made at a police station. The situation is different in cases where blood or urine are requested.
I understand that some police forces in the UK are currently piloting a new evidential breath testing device, the Lion Intoxilyser 8000 which is intended to eventually replace the Lion Intoxilyser 6000UK which is currently in use. However, I have been unable to find any confirmation that this device has yet been type approved by the Home Office, which it would have to be before the police could rely on it in evidence.
I have spoken about your case to a forensic scientist who is an expert in intoxilyser procedures. From a toxicology and practical point of view, the machine could be set up in the back of a police van and providing the machine is type approved and properly calibrated there would be no reason to doubt the relaibility of the machine. There is a potential legal argument as to whether the vehicle you describe fulfills the definition of "police station" or whether there is some other legal authority that dispenses with the requirement for the request to be made at a Police Station. It may be that the police in your area were involved in a drink driving campaign or pilot scheme and will argue that the mobile unit falls within the definition of "police station" under the road traffic act. This is a moot point and will depend on a number of factors.
It is certainly an unusual situation. If you have been charged I would recommend you take legal advice from a specialist drink driving solicitor before you decide how to plead. It sounds like the case raises some novel legal arguments that may not yet have been tested in court. If you want more help then feel free to email me direct.