Avoid the scandalously high cost of making a phone call from your hotel room

Use Skype (and an internet connection) to make and receive free voice and video calls—or have your friends and family Skype your cell in the U.K. for 10¢ per minute or $1–$2 per month.

Emergency numbers for police, fire, and other safety issues in the U.K.

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.

How to use a cell phone (in British: "mobile") in the U.K., whether using your own, or buying or renting one to use while you travel.

How to make phone calls to and within England and the U.K.

Rent an international cellphone or WiFi hotspot to use while traveling.

Almost as cheap as renting—and probably a better idea if you plan to travel abroad more than once in the next few years—is simply to buy your own world phone with an international plan.

It's easiest—but potentially pricey—to use your own phone to travel; just be sure you sign up for an international roaming plan before you leave to avoid huge data charges.

Keeping in touch while traveling—Cellphones and Skype, mail and roaming fees, and how to call to, from, and within the U.K.