Is this a sea slug?

Is this a sea slug?

Observation - Is this a sea slug? - UK and Ireland. Description: Two different animals both about 2cm long. The first picture is a different individual to the other two pictures. Last week there were hundreds but this week these two were all we could find. I'm assuming they are the same species.

Two different animals both about 2cm long. The first picture is a different individual to the other two pictures. Last week there were hundreds but this week these two were all we could find. I'm assuming they are the same species.