I was just there and can tell you, do not use US Providers Intl Plan.
I got a "Three UK" sim card when I was in London.
Purchased 20 pounds top up at a Tesco (Walgreens in London)
Go online and switch to "All in One" package
12GB of Data and 3000 texts.
This was only good for 1 month, but the "All in One" has the "Feel at home" portion that she needs.
My phone number was UK, but the data worked in France and Netherlands. I only got 3g in France and Netherlands, but never had a problem.
The only problem is that she would have to purchase the top up credits in advance in England before leaving.
Without a UK address on a credit card, you can't purchase online.
Three UK Feel At HomeFor Calling, utilize Whatsapp. If you don't know what that is, get it.
If she doesn't start in London, what is her first country?