I've travel London and throughout Europe. Here's what I do, which ends up costing much less and works flawlessly. Carrier international services can be expensive when overseas.
I always utilize my Magic Jack Phone App. I transfer my Verizon number to my MagicJack Phone App and upon arrival in Europe I get me a local carrier SIM Card which has significant amount of data (3-6GB depending upon the length of time there). This way my Verizon phone calls are transferred to my MagicJack App on my phone and it rings me there. Works flawlessly for me.