Starbucks in the UK does offer free wifi if you have one of their pre-paid cards. The card its self is free so just load £5 and you'll spend that on the drinks you consume whilst using your free wifi.
TBH I've never found, as a tourist, free wifi to be that much more prevalent in the US than it is in Europe. You'll find it in the same sort of places (coffee shops, museums, hotels, even pubs,) with the same mixture of totally free access, free-with-purchase and chargeable.
If you come back with a more detailed itinerary (countries and cities) I'm sure the gang will have some specific locations. In the UK Starbucks, McDonalds and Wetherspoons are a good start.