Confidence: I'm as sure as I can be.
Notes: Gastroclonium ovatum
The egg-shaped branches, narrowed at insertion are characteristic for this species. It often occurs as an epiphyte on Corallina officinalis, as well as on rocks. Have a look at this example that I posted and see what you think.
Welcome to seaweed ID, and do keep posting. I think this is a good book: Bunker, Brodie, Maggs & Bunker 2010 Seasearch guide to seaweeds of Britain & Ireland Marine Conservation Society.