Trick question! Somerset is pretty warm!
Here is the range of the American Alligator. Wilmington, NC is clearly within the alligator's range. Here is Wilmington's climate, and here is Bath, the closest city to the Somerset levels with record high and low temps on Wikipedia.
The mean low in Wilmington is 2 C and in Bath 1.9 C. The record lows are -15 C and -14 C, respectively. So it is not necessarily 'too cold' for alligators in Somerset.
Now, Alligator aren't really very active below 20 C, so cool summer temperatures in Bath aren't very conductive to crocodillian life forms. On the other hand, some hold the opinion that crocodillians evolved from a warm-blooded common ancestor with birds and dinosaurs. If that is the case, it isn't much of a stretch at all to imagine one line of alligator-like creatures surviving to the present with improved ability to thermo-regulate, if not full on endothermy.
The important point is that swamps in Bath, like the swamps near Wilmington, do not freeze hard in the winter. That is the key development that would make the alligator life-style untenable; they'd have to get out of the water for some months of the year and hide/hibernate. For young alligators, the pressure from medium sized carnivorous land mammals (racoons, badgers, foxes, coyotes, etc) would destroy them quickly ( 1 year old alligators are in the 10-18 inch long range; in the cold they'd be easy pickings). But with young able to stay in the relative safety of the water all year, the species becomes more viable.