Thanks for the reply --- sounds like it does indeed work unlocked internationally, as was advertised. I assume your Three sim which works in the US must operate like the Verizon Global services plan, where I would leave my VZW sim in place when traveling and just have my phone "turned on" by VZ to operate in Europe. Do you find that to be as satisfactory/inexpensive as swapping out for local sims when you travel?
I think you misunderstood Sarks. The Three / Hutchinson sim would not work in the US, and as Dpham00 says I would be very careful not to use a Verizon sim in Europe. However I am in the UK and don't know what international roaming deals they can do for you.
My phone is on a different network here (Vodafone /Talkmobile) but I use a Three data sim monthly for a mobile Wifi / MiFi dongle.
If you are spending a lot of that time in the UK - Three had long been the best value in data and preloaded disposable sims, especially via Amazon UK. Recently their preloaded prices have increased, and the EE network are better value. Currently on Amazon (though this may be a short offer) you can buy an EE (4G) sim for an incredible ?15 which has 10gb over 3 months, and includes ?15 of phone credit for a call and text bundle. If I can also put a link later I will.
Again though while Three include some free European roaming I would think EE would limit you to the UK. At least with pre-paid you won't come home to a large bill. You could still afford another prepay sim from a European wide network.
**added for any UK visitors, also look at 'Three':
Via my Note 3 on AC.