Can anyone confirm if this is, in fact, thongweed?
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Yes I think it is thongweed - the fronds are flattened and forked. Chorda filum (mermaids tresses) is a similar long thin seaweed but it is cylindrical and hollow and not branched.
Lat/Lng: 56.18583, -2.81823
OS grid ref: NT493995
Found among drift material at incoming tide.