if I am satisfied that no intention to drive probably exercise discretion, words of advice, tell Mr Bloggs to give keys to Mrs Bloggs, lock them away in a cupboard etc. etc. then move on to a different job. If I am not satisfied then arrest, EBM procedure & interview when sober about the intentions.
(....)
I will say Bloggs is on very dodgy ground & in answer to your first post [about a motor home owner parked up and with the seat facing away from the wheel] the law is quite clear. An officer would be quite within his / her rights to arrest.