Don't go with EE. We use them for our company mobile phones and we all struggle with reception issues...and that's in central London!!!! O2 and Vodaphone work fine but in exactly the same spots, EE has no/limited reception. Very frustrating!

I'd also avoid 3 as they're very easy to sign up with for PAYG etc but their customer service is appalling, and they will quite happily screw you if you give them the chance.