I can confirm from experience as a US-based traveler that my Google Voice number works well in Scotland. I still have a SIM that I purchased from http://giffgaff.com, but there are also SIMs available for UK use through Amazon. You can purchase a pay-as-you-go plan through these companies that allows you to get various amounts of data and minutes that are good for 30 days. Some of those plans are as cheap as £5. For your limited purpose, Google Voice may be all you need, but it is nice to be able to make a local (or even an international) call when wifi isn't available.