All the standard UK contracts are pretty pricey to make calls from Europe, and insanely expensive outside.
The best thing to do is use an unlocked phone, and get international sim cards. To get an unlocked phone, buy it not on a network. International sim cards you can buy either from dodgy small phone shops, or online. Sometimes it is best to get them in the country you’re going to, it is worth checking on google for the countries you’re interested in.
If you’re going to Europe, almost all UK phones will work (dual band ones). Other places you could do with a tri-band or preferably quad-band phone, which rules out the cheapest ones, but you can get a slightly more expensive one. Or if you already have a phone, find out if it is multi-band and if so get that unlocked (shouldn’t cost masses at a dodgy phone shop).
Joe