Last year drought in the UK and Europe reduced pasture and cereal production, making feeding animals more expensive. Much milk was diverted from butter to cheese-making. (See how many new cheeses there are on offer these days?). Instead of having a butter mountain, EU production of butter only just met consumer demand, and this meant suppliers could charge more.
Interestingly, regarding feedstuffs, Anchor butter used to say on the packet that their cows were 'fed on grass the whole year round.' This means the price for Anchor butter shouldn't have gone up, but it has. It's risen in line with prices for European butter, and, curiously, the 'grass' message is no longer printed on the packs. Hmmm.