As juansheetisplenty says, Lebara is very good and do deals with international calls and texts at a very low price.
However (there's always a however) they use Vodafone networking which, in our area, is very hit and miss. I had to stand on our balcony to get a signal but I understand this is a problem in our area as Vodafone gets the best coverage report in Spain. Movistar was also a pretty poor signal area and, if buying a PAYG, you have to keep using it in a set space of time or they disconnect the number which means topping up before the time runs out.
We switched to Llamaya which is a PAYG and just goes on whether its topped up or not. It uses Orange which, in our area, is a better signal.
However, if you're mainly phoning UK, then juansheetisplenty's suggestion is one of the best as they have a €10 a month contract with unlimited UK calls and texts providing you are in an area which gets the signal on a reliable basis.