Use T-mobile when traveling ★★★
T-mobile offers the cheapest international roaming—it works in 140 countries and costs as little as $10–20 per month for voice and data.
T-Mobile phones on many plans actually work in 140 countries, including the U.K. (and most of the rest of Europe).
Better still: voice and data plan start around $10–$20 per month—though they (like most providers) are constantly tinkering with and changing the names and details of their plans, so I can't tell you which one to pick; just shop around on the site.
I carry a small, cheap Android tablet with a T-mobile SIM in it just for this purpose—and, since they no longer lock you into a two-year plan, I can sign up and drop one of their cheap data plans whenever I need one just to travel, then drop it when I get home. Nice.
Without an international plan, calls can still be pricey, but data costs nothing extra, so you can still Skype.
Just keep an eye on that data usage, and maybe spring for more GB on the month(s) you will be traveling.