They aren't priced on that site, the manufacturer doesn't sell direct. The UK phones will work in the US yes, they are all either Tri or Quad band. If you bought from the UK they would be network locked, which can be fixed with some effort. If you buy inside the US they'll work regardless. Try e-bay for some prices. The best models are more or less the JAM, k-jam, jasjar and SP5.

T-Mobile is the number 3 provider world wide. They have won the JD Powers award 4 years in a row.

You'd be surprised... Majority of world userbase for the company is in US. T-Systems are second biggest mobile operator, behind Vodafone.