Be aware that some SiM cards allow data but prevent VOIP (voice over internet) I use http://www.nordvpn.com to get round this, plus it also allows the iPad to access UK TV services like Sky Go, BBC iPlayer and ITV player. You can buy by the month, or pay $48 per year. I have no association with nordVPN other then being a satisfied customer.
There is also plenty of free Wi-Fi around. McDonalds, Panera bread, most airports etc.
Finally, 3 UK offers in bundle calls and data in the US for no roaming premium, so you can use your 3 phone in the US as though it is on a UK tariff. Note if you go over your UK minutes and data allowance, the high roaming fees kick in.