@ Jamesdevice,
we don't have a dissident population like China
The United Kingdom is an exporter of terrorism. This is why the USA has such strict security on people coming from the UK. Think of UK mainland attacks such as Glasgow Airport or the 7th of July 2005 London bombings. Also think of the fact that the UK is under severe stress from the economy, with many dissatisfied indigents. Then there is the issue with border security being a total failure, leaving us with somewhat porous borders that are perfect for allowing undesirable persons in to the country.
I would also like to raise how difficult it is for UK security to control any one or any thing within the UK. The laws do not allow for it.