Best SIM Card for and iPhone 5?

I'm traveling to the UK Over the summer and I'm trying to figure out what the best SIM card would be for my iPhone while I visit would be. Any ideas. My carrier is Verizon, if that helps at all.

EE its the largest in the UK

This assumes that your Phone is network unlocked and able to take any SIM card.

O2 Sim -
Take the international SIM (on PAYG - so no contract)
Internaltional SIM allows UK calls at normal O2 rates
BUT best of all, cheap call (4p/min) to US and Canada

top it up, say 15 pounds
and they often give you 'free' data allowance
even if they don't
you can also buy 500MB of data useage for 5 pounds.

for canada,
http://www.o2.co.uk/internationalfavourites - international Favorites
you can even buy a months international option (10 pounds)_.,
and that gets you
3000 international mins
and A "call me" number…
you choose a HOME (US) number that someone calls in the US,
and your UK phone rings!
i.e.everyone at home can call you as a local also!
(and a separate 3000 mins for that also)

buy the 15 pound international option
and you get more mins, and more call me No's (you can have more than one foreign contact No - different cities or different countries)

http://www.o2.co.uk/international/intsim - international SIM

All the PAYG (pay As You Go) tarrifs (international is the same as O2 unlimited)
http://www.o2.co.uk/tariffs/payandgo

edit:
if you are not worried by calling home or home people calling you
then the GiffGaff SIM is a worth while option (actually owned by O2!)
http://giffgaff.com/