TL:DR – The bridled guillemot is a polymorphism (not a sub-species) of the Common Guillemot (Uria aalge) found in the North Atlantic region.
Guillemot is the common name for various auk-type seabirds (Charadriiformes). In the UK, there are two genera commonly seen: Uria and Cepphus. Common Murre, also known as the Common Guillemot (Uria aalge) is a large auk with circumpolar distribution. It spends most of its time at sea. It breeds on rocky cliff shores or islands. The Bridled Guillemot is a polymorphism of the species found in the North Atlantic region.
My photo shows a Guillemot at RSPB Bempton Cliffs on the coast of the East Riding of Yorkshire. If I told you that there is a species of guillemot known as the spectacled guillemot, you might imagine that this is she. However, this is actually a bridled guillemot. It’s not a distinct species but a genetic polymorphism of Uria aalge, the common guillemot (aka the common murre). This strain has thin white circles around its eyes that stretch back as a thin white line. By contrast the spectacled guillemot is rather distinct looking and has thick white circling around its eyes and no “bridle” and is Cepphus carbo.