This again!
In order for a species to survive, there needs to be a minimum viable population:
https://en.wikipedia.org/wiki/Minimum_viable_population
Anything below that and the species will die out, mainly due to in-breeding which will render the individuals infertile.
It is just about feasible, I suppose, that a colony of creatures lives in the North Sea and occasionally a lone individual finds its way into Loch Ness via the Caledonia Canal, though how they traverse the lock system without being seen is anyone's guess.