Assuming it was insured to be driven by any driver with the permission of the policy holder in the UK of course. OP says they have an English version of the policy now, but hasn't confirmed this is the case.
If the certificate isn't accepted as valid cover for driving in the UK by a UK licence holder, it would be best to get the driver to post here directly. They are in receipt of the COFP and the invitation to accept 6 points, a financial penalty and crap insurance quotes for the next 5 years, not the charity, and as such they will be the one having to deal with it. If it goes on that the policy didn't cover travel in the UK, there may be an argument for SRNTE for the driver if they can prove they were sufficiently misled to believe they were insured to drive the vehicle but this will need to be argued in court.