Rattus rattus, the black rat, quite different to our pet rattus norvegicus, and far smaller.
Rattus rattus are actually VERY rare in the UK now, and are considered one of our most endangered mammals.
As a point of interest, rattus rattus is the rat species commonly (and incorrectly) linked to the plague, NOT rattus norvegicus, the species our pet rats and most of our wild rats are.

Rattus rattus is smaller, more 'mouse-like', more agile, great climbers, bigger ears, longer tails, and are generally less robust than rattus norvegicus. They've never really been successfully domesticated, unlike rattus norvegicus.

This skull is tiny, about an inch and a bit long.