At which point he could have disposed of evidence, refused to be interviewed, etc etc. This is exactly how the legal system in the UK works - you arrest people

At which point he could have disposed of evidence, refused to be interviewed, etc etc. This is exactly how the legal system in the UK works - you arrest people at a relatively low burden of proof, and that gives you the power to detain them for a few hours while you question them, search for evidence, etc. You can't do any of that if you go over and "ask"