I’m pleased that you seem to have found a solution Neongreen, but I have only just noticed this thread and would have made the following observation.
I do 80,000 kms a year all round Europe in my car and, although I don’t legally need to, I keep more or less to the same restrictions as I had to when driving a wagon, and I would love to have a tacho installed. With no worries about legal restrictions to trip me up it would nevertheless serve as evidence of innocence if I ever needed it. My excellent mechanic, who is a time served truck electronics man, said I can have one, but at a price, and where the hell would I put it?
So, in your situation, where there is no legal requirement, why not try to buy a job lot of cards from the UK, and use it in a defensive mode? 