Residing in Dallas, TX, USA, we definitely enjoy the abundance of Tier 1 fiber options. Accessing the service only a few hops from the data center provides instant response from the servers.
Please consider the geography and availability of reliable, fast dark fiber. This is what separates a Dallas, TX-grade performance from a St. Louis or Kansas City-grade performance.
UK bandwidth seems more affordable (possibly due, in part, to the short distance in latitude of the UK, relative to the US). Most European residents will benefit from a UK connected service, much like North Americans will from US connections.
If you are a web host, it’s obviously advantageous to be connected to both regions. Even small hosts will benefit from housing a server in the opposite continent for the purpose of nameserver redundancy.
@Chris Worner It is a good idea to consider geography when you asking any questions like "What is the difference between UK & US datacenters?". US and UK have different jurisdictions and different business environments. Even the power standards are different which changes the power distribution, measurement and costs. Tehre are many other differences and it owed take pages to outline them.
The first and the most obvious one is that UK and US and on different continents, I hope that you have a a world map, it would not take a lot of time to explore it for the purpose of this discussion. Then if you really mean that you "can not distinguish differences between these countries", then with all the respect I'd say that it is hopeless
PS: I saw that ForwardWeb - Brandenandradwebhosting have taken your question seriously and have been very helpful by making a meaningful suggestions, by answering to a meaningless question.