Confidence: I'm as sure as I can be.
Notes: This green seaweed is growing on a piece of fucus frond; when I collected it I thought it was a filamentous green algae, possibly Cladophora .
However, under the microscope, the fronds are not filamentous, but narrow (less than 0.5mm), branched, flattened, tubular and one cell layer thick. The cells are not organised into obvious rows and have a single hood-shaped chloroplast.
I have chosen U. compressa as Brodie et al say “This species can be difficult to distinguish from branched U. intestinalis, but (the latter) generally branches only under reduced salinity whereas U. compressa is not found at very low salinity.” Preston beach is not an area of low salinity.
Green Seaweeds of Britain and Ireland, 2008. Edited by Juliet Brodie, Christine A Maggs and David M John. British Phycological Society.