A small-white butterfly feeding on Hogweed flowers recently. This was the usual form of the plant, but there are others in the area (Pontypool, south Wales) which could be a narrow-leaved type or hybrid, or not hogweed at all. Sample photo below:

“normal” and narrow-leaved hogweed together

There is a photo of three potential variations here, and an album of photos here. I’m informed the narrow-leaved type, on the right in the above photo, is widespread in the area (see comment). This has the scientific name “Heracleum sphondylium ssp. sphondylium var angustifolium” I believe.