Abundant in rock pools
No interactions present.
On the face of it, this looks too 'robust' and perhaps 'luxurious' to be prolifera. I may have some to hand which complies very well to this description https://seaweedindustry.com/seaweed/type/ulva-prolifera though mine is more finely branched.
Alan is quite right to be reserved but a much safer ID might be Ulva.
An old post yes but it will surface again soon because I am on to the Ulvales
Lat/Lng: 50.0238, -5.097
OS grid ref: SW782183