It had 8 primary tentacles with white translucent tips. The fishermen reported large schools of these jellyfish and hadn't seen them before. Photograph Cyanea lamarckii. Both Cyanea jellyfish will occur together sometimes. Common off Scotland.
Most reports say it is innocuous.
Not enough information, but probably similar to Cyanea capillata . 26 June 2001 Hundreds of jellyfish were seen off Sandgate, Kent. They were about 12 cm in diameter but have not yet been positively identified. This could be the uncommon species Cyanea lamarckii. Report and photograph by Philip Saunders I think that this jellyfish is Cyanea lamarckii .