Usual reason for holes in tubers, in Britain, is keeled slugs. These live in the soil and burrow into the tubers. Then other things move in, like millipedes or woodlice.
Or, it could be leatherjackets, they too bore into tubers.
Cut worms are another possibility too.

my first thought were small slugs too Owd.

Are there wire worms in the UK?

Yes and they do burrow into potatoes too,Sorry forgot about them.
Larva of click beetles by the way.

thanks