cell phone/ sim card roaming

It depends. Some companies' SIMs won't work outside their country of origin. The worst that could happen, though, is that it wouldn't work and you would have to buy a new SIM. You should ask wherever you buy your first SIM to make sure that it will work outside the UK.

megan, If you buy SIM cards in the U.K., there should be no issues using them in other countries in Europe. However, make sure you're clear on how to "top up" in other countries. Some networks may offer an "automatic recharge" option. If you think there might be a possibility you may have to use other SIM cards, be sure the phones you buy are "unlocked". One other important point to consider is that if you buy phones in the U.K., they'll be equipped with the usual gargantuan Plugs. You'll need Plug Adapters to charge the phones in other countries. Another option you could consider would be to buy a phone from one of the "travel phone" firms such as Cellular Abroad, Roam Simple, Call In Europe, Telestial or Mobal prior to your trip. That would provide consistent rates throughout western Europe (as long as you're in Zone 1 countries), with calls usually charged to a credit card so no worries about topping-up or running out of minutes. Happy travels!

A new SIM card will reduce per minute charges. As far as I know, your London Sim card will get you cheap rates while in Britain but you'll pay roaming charges (though not as bad as US phone companies' roaming charges) when you cross onto the continent. If you aren't making many calls, just keeping that Sim card may be your best bet. If you'll be in another country for awhile, or making a lot of calls, it may be worth it to pick up a new Sim card - you really have to weigh the options for your particular trip and usage to know which is the best deal for you.

megan, One other point to mention. If you use a SIM card in countries other than where you purchased it, the roaming rates will be higher, but not excessively so. The E.U. has "capped" roaming rates so they're not as high as they once were.

In addition to everything said above, check the website (or ask at the store when you buy the SIM) for deals on European roaming. For instance, T-Mobile UK has "Euro Boosters," wherein you get more credit than your are paying for, but only if you do something special. For example, if you text "Euro 5" to 411, you spend 5GBP of your credit, but you get 7.50 GBP of credit to use for European roaming (50% bonus). Since the UK is a very competitive market for cell phone plans (just be sure to use the term "mobile" instead of "cell" there), I'm sure other companies also have deals. Here's the info for T-Mobile (scroll down for "Euro Travel Boosters") http://www.t-mobile.co.uk/shop/mobile-phones/price-plans/pay-as-you-go/boosters/

If you only expect to use them for emergencies or if separated if you have AT&T cell phones in the US you can add the international texting option for only $10 for 50 outgoing texts within a 30 day period. Incoming texts are free. It works great and I have used this option all over Europe as well as Guatemala last week. That way you have your own phone, you know how to use it and family can reach you if necessary. If you have an Iphone you just need to turn off Data Roaming so you do not incur extra charges. Its really easy and cheap.

If you only expect to use them for emergencies or if separated if you have AT&T cell phones in the US you can add the international texting option for only $10 for 50 outgoing texts within a 30 day period. Incoming texts are free. It works great and I have used this option all over Europe as well as Guatemala last week. That way you have your own phone, you know how to use it and family can reach you if necessary. If you have an Iphone you just need to turn off Data Roaming so you do not incur extra charges. Its really easy and cheap.

The Talk Abroad SIM by National Geographic is made exactly for trips like yours. It's only $0.69/min out to US, free incoming and has both US and UK phone numbers ($.30/min in to US line). Really, it's a simple solution. Cellular Abroad has it.

Megan, my experience is with T-Mobile U.K. Yes, you can buy a SIM from them and use them throughout Europe. I have done so in France, Italy, and Germany. Check out the boosters packages as another person recommended. You can see all of the rates on T-Mobile U.K.'s web site. The advantage of going with a British cell company is all the information is in English.