You will need a global phone that is GSM in addition to CDMA, as the UK uses GSM technology. If your phone isn't global, you can rent one from Verizon. You also need a SIM card and have to call customer service to activate your SIM and have them enable international dialing for you.
You do not need to subscribe to the $4.99 global value plan. But if you plan to make or receive any calls during those two weeks, it'll lower your rate from $1.29/minute to $.99/minute. The global plan does not discount text message rates, which are $.50 to send, $.05 to receive. But here's the important part: If you send or receive picture messages ($.50 to send, $.25 to receive, plus data), add the global data plan. It's $25 for 100MB, but without it you'll pay $20.48/MB. You will be credited for the unused data when you cancel.
I traveled to the UK in April and it all worked fine. All I had to do was dial *228 option 2 before leaving the U.S. Enjoy your trip, and let me know if you have any other questions!
Something you also need to know is that when you add a Global data plan, it is a monthly plan. So, if you add the plan in mid billing cycle you have two choices, have the plan applied on the date you need it or have the plan retroactively applied to the beginning of the cycle. If you apply the plan on the date you need it, you will only get a prorated amount of date. For example, if you apply it on the last week of a 4 week billing cycle, you only get 25 MB of data and not the full 100 MB. If you retroactively have it applied to the beginning of the billing cycle, you get the full 100 MB of data but any data you have used in that entire billing cycle counts against the 100 MB of data allowed. Either way, unless you have the plan applied on the first day of your billing cycle, you are not getting 100 MB of data for $25. I guess the only way to keep from getting ripped off is to plan your trips to coincide with your billing cycle. Bear in mind that Verizon chooses to apply the plan in this manner so it doesn't have to be this way. I also recommend you check your bills for the next few months very carefully after adding or removing a Global data plan.