From the frilly arms & bluish tinge it can only be the barrel jellyfish Rhizostoma pulmo, see eg http://www.nhm.ac.uk/natureplus/message/47931 . It's uk's largest jelly ( the Lion's mane can grow larger but not in uk.).
Despite it's size, it is "microphagous, employing mucus to trap fine particles", according to Hayward's SeaShore '96. Their sting has been described as 'mild' or 'not strong enough to harm humans' in the papers, but I was foolish enough to try & get 1 back in the water to photo it, wearing gloves & using some netting, & got stung on the arms, which was equivalent to a nettle sting & lasted as long.