I had two nasty surprises on our Edinburgh Fringe visit with UK mobile service providers that I wanted to share with other visitors from the USA. Briefly, don't trust what the vendor "3" (Three) says and be aware that calls using the vendor "MaxRoam" take one to five minutes to connect. The details follow.
The company "3" (Three) lied to me. I needed to have a "Personal Hotspot" to provide Wifi so we could use our older iPhones for Internet and apps even though they lack Europe-compatible mobile chips. Then we could use apps like Edinbus to see bus stops, routes, and arrival times and use Skype for phone calls. We planned to carry our new iPad, which has a Personal Hotspot built in, and which "3" advertises they support. But the Hotspot didn't work when we put in our "3" SIM. The manager at "3" on Princes Street finally fessed up--only customers with contracts get the Personal Hotspot! (Visitors can't get contracts.) Even just using the iPad directly, "3" gave no signal at several Fringe venues along George Street. Please learn from our experience with this untrustworthy provider.
MaxRoam at least works--eventually. I wanted SIMs for our older, "dumb" Euro mobiles as backup in case we had a problem with the iPhones (as we in fact did). We had used the provider "Orange" with complete satisfaction in 2008, but this time we needed phones that worked the moment we arrived, so I bought MaxRoam SIMs from the US. Unfortunately, every time you place a call using MaxRoam, there is a one to five minute delay while the phone searches for a VodaFone roaming connection in Edinburgh! On our next visit we will go back to Orange.