I've always used calling cards from the likes of cloncom (
linky). The idea is that you buy a card from t'internet (or more likely an access number and a pin code) which you use to make your calls. You can either use a toll free access number or a local rate number to access the service. You generally get more minutes for your money if you use the local access number. I've made it sound more complicated than it really is. The bottom line though is that you can get call rates of less than 3p/min.
hope this helps.
ps..forgot to add that you'll get around 500 mins for about £12.