X-ray is not that unusual to check for contraband at customs, although it depends on the country whether or not it is a routine practice or requires suspicion. Perhaps the UK does so on domestic mail as well but that would be surprising to me, it's pretty high cost to do at large volume. These X-ray machines are the same type used at airports and usually produce either a synthetic isometric 3D-type image or two 2D images at 90 degree offset. The resolution and sensitivity are both nowhere near high enough to read text on documents, they're primarily oriented at recognizing firearms and very dense objects typical of drug smuggling. Much of the data modern X-ray parcel scanners produce is synthetic as well, based on machine vision and multi-exposure methods, so the display tends to suggest that the data they collect is of higher resolution than it actually is.
You would struggle to determine whether a page is folded A4 or A8 with one of these machines, reading text is far beyond their capabilities.
