Ah ok, I've been meaning to pay the Dungeness area a visit again its not far from me really, i quite like the strange vibe the place has to it.
I think you are right about it being a moon jellyfish, the one you found looks quite similar to my one and on the second picture you can just about pick out the four semi circular rings in the middle. Looks like they are quite common in our seas but nice to get a close look at them