Cresswell Beach Ugly Betty Jellyfish

My grandson Oliver found this jellyfish in a very clear, warm sea water pool on the beach between Cresswell and Druridge bay, Northumberland.

It was about 14 inches dia, about 6ins deep, pulsating or breathing, it looked like a cross between a crazy hat and a trifle..it had about 8 triangular, red striped flags around the perimeter with tentacles beneath, there was a pile of yellow growth on top that looked like mashed custard or scrambled egg and a pile of red strawberry jelly type stuff on top of that.
Scary..we really thought that it might jump on us..what was it please?

Kind regards
Susan