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Brown seaweed with a midrib - probably a Fucus (wrack) but more info would help. For example, size (always useful - even a coin alongside helps); where on the shore - upper, middle, lower? A piece spread out to see details of margin and any reproductive organs. Have you got any more photos?
it was growing about mid-shore and grew in a rugby ball sized clump but it thought, as it seems pretty common, most people would be able to tell it from its general impression of size and shape so I didn't think further pictures where necessary. hope that helps.
Lat/Lng: 50.2444, -5.3251
OS grid ref: SW630435