I've seen Graham's milk in England, but never the butter.
I believe Yeo Valley butter is from grass-fed cows (and ones specifically on organic pasture). That seems fairly widely available, e.g. in Sainsburys.
There's also Rachel's Dairy. That was a Welsh-owned firm, but has now been bought our by a U.S. company. However, it still seems to be Welsh cattle grazing
Rachel's Organic - Milk, Cream & Butter - Semi-skimmed Milkfreely in pastures rich in lush green grass and clover, untreated with any artificial fertilizers or pesticides
The Rachel's products are available in Wales and in England -- e.g. in Waitrose.
I think on the whole cattle (and other livestock) are less intensively farmed in Britain and Ireland (and New Zealand) than in the U.S. They tend not to state "grass fed" on labels for the British market, however, because consumers in Britain aren't specifically looking for those words -- I guess on account that Paleo, and awareness of the whole omega-3/6 issue, has not made as big inroads in Britain as it has in the U.S. Kerrygold, as far as I know, not only use milk only from grass-fed cows, but say they only use the summer milk (which has a higher vitamin content) for the butter. I don't think Kerrygold bother to state that the cows are grass-fed on the labels in Britain though.