I'm going to London and Liverpool for four days at the end of this month and have an unlocked iPhone 6s Plus. I looked at AT&T international Passport plans and the rates are insane. I've read that prepaid Vodafone SIMs work very well in both cities. Should I buy the SIM now, if that's even possible, or wait until I get to Liverpool? Any other suggestions for better/cheaper service? I know it's only a 4-day trip but I really want to be reachable from the US by my relatives and pet/house sitter.