Best prepaid carriers in the UK and France?

Hello fellow Vergins! I will be travelling to the UK and France from America later this year and need help finding the best prepaid GSM carrier for my Nexus 5.

I have never had to buy prepaid service in Europe but, to my understanding, there are no roaming fees between EU countries. If this is true, I need help picking the best carrier for these two countries, with good service in London, Paris, and Normandy being key.

Any help would be hugely appreciated!