"If there are 1.3 million virus infections reported to the police, there must be hundreds of millions of actual infections. In my country, I'd expect reporting a computer virus to the police to be at best a pointless exercise. Are UK cops more diligent? Or are expectations higher?"
This has nothing to do with reporting crimes to the police. These crime statistics are produced from the Crime Survey for England and Wales (formerly the British Crime Survey), which is an annual survey of 50,000 people asking about what crimes they have been victims of, in face-to-face interviews so avoiding a lot of the obvious criticisms of less thorough surveys. The entire point being that many crimes, such as computer viruses but also more serious things like domestic abuse, aren't often reported to the police and so don't appear if you only look at recorded crime.
There are still plenty of criticisms that can be made about it, but it's generally considered to give a much better idea of the levels of crime than pretty much any other measures.