The long feathery part is, I believe, a large hydrozoan. Washed up on beach after a stormy night, see http://www.ispot.org.uk/node/306080 and http://www.dorsetwalks.com/reports/portland.htm
No interactions present.
Another post badly consigned to History without comment or agreement - despite over 320 reads.
I suspect you are right with Gymnangium montagui though is really is not easy from just one photo (I have agreed - it's LIKELY to be this!)
A western, not specially southern, species - see the NBN map
Epiphytic and lithophytic
Light yellow feathery fronds
Clearly just alternate and not precisely opposite - see Aglaophenia cupressina
Not the more common Halecium halecinum
The dorset Link has long since eroded but there are others -
Oh, and yes to Hornwrack
Please add the tags Marine and ProjectM1
THREE MARINE PROJECTS
while I was following up this id....
EDIT 3.3.17 I agree with Bob that this hydroid is probably one of the Aglaopheniidae.
Three British species look, in illustrations and photos, as if they have alternate branches directed to some extent forward. Lytocarpia myriophyllum, Aglaophenia tubulifer and Gymnangium montagui.
Ids of hydroids are easier if details of the individual hydranths are visible. However we don't have that here, so I have not agreed to a species.
Question: Can we assume that as the hydroid colony grows longer that the length of the side branches is limited? I couldn't find confirmation of that. It's not always clear to me if the various lengths given in the literature represent average or maximum values for our British waters.
Lat/Lng: 50.571027310297, -2.4664306640625
OS grid ref: SY670746