Galaxy S4 in the UK and the US?

[Whitebread]

I'm looking to buy an S4 that I can use in the US and UK/Europe (when I'm traveling). It seems that Samsung does not make a phone that supports 4G frequencies in both the US and England. So, I'd be okay buying a phone that supports 4G in the US and 3G in the UK/Europe.

To that end, I see that the SGH-i337 supports all the 3G frequencies listed for the four major UK carriers (Vodafone, EE, O2 and Hutchinson 3G). However, I have two questions.

First, Does the carrier need to support all the 3G frequencies the phone supports in order to function? I'm guessing that the phone only needs to support 1 frequency the carrier supports but, additional frequencies means more bandwidth/greater flexibility.

Secondly, I'm not familiar with the frequencies used in continental Europe so, I'm not confident that a 3G phone that works in the UK will work in western Europe and Scandinavia. Does anyone here have any experience traveling in those areas with a 3G phone?

Thanks