I had the messge below from SODAQ but must admit that I have had no luck.
Spoke directly with Telefonica and their coverage seems to be in the East of the UK at the moment.
I haven't tried recently as have had to move on with other things but if you get any joy I'd be pleased to hear.
I sent some kit to a colleague in the South of France and there was no connectivity issue there and also in Spain the coverage seems good.
It looks like we are the poor relatives at the moment, but hopefully that will change quickly.
Good luck !
For most cases: If you have 4G/LTE coverage, you also have NB-IoT coverage.