US Hosting Companies can charge less as the biggest costs for hosting companies are power costs and salaries, both of which are much less in the US.
Bandwidth is also cheaper in the US compared to the UK where BT own most of the UK backbone and charge hosts through the nose to allow their traffic to go across their network.
Even software and hardware costs in the US are much lower.
This means that UK hosts are not playing on a level playing field with US hosts.
The problem with using US hosts is that you have a US IP meaning local UK searches will not rank your website as high as those with a UK IP address.
There are also very different laws in the US like their DMCA which means that as soon as the host gets a DMCA notification they have to immediately pull the plug on your website.
There are pros and cons of hosting in both the UK and US and the lower price is just a small part of the bigger picture.