I am back from my trip and the service generally worked OK. It seems in some cities it took a little time to "settle and connect" upon landing. In the US, it actually went among AT&T, Verizon and T-Mobile. I did not make a single call and only used 2,5 GB of the 10 GB I bought. So, the £10 4 GB plan would have been enough. Still the £15 10 GB plan was not too bad and I didn't have to worry about hitting the limit.