Butterfly Conservation (which is the leading British charity dedicated to the protection of both butterflies and moths) doesn't list any all-red species on its website. The nearest might be a cinnabar moth:
https://butterfly-conservation.org/moths/cinnabar
If you're interested in trying to identify butterflies and moths in future, you might like to bookmark these two links:
There is an all red moth it has a yellow and black caterpillar and is conspicuous because it tastes horrible to birds so a bird will only ever try one and never again. The caterpillar lives on ragwort. I can't remember the name of it.
I'm pretty sure it wasn't a cinnabar. I could see no black or other pattern on it. It seemed to be solid red/crimson/scarlett. I'd thought it was a bit of red paper fluttering in the breeze when I caught it out of the corner of my eye.
I said rowan was right because there are no red butterflies in UK (unless you saw an extremely rare visitor) and people often mistake cinnabar moths for butterflies, but I could be wrong I guess.
I've just googled cinnabar moth and the pics that came up make them look a lot different to the ones I've seen, there seems a lot more black on the ones in the pics.