Fair point. But I'm sure it will work the same way most tax or motoring laws work.

When you have proven this to be true, possibly in court, they may accept it. Until that time, they'll seize your goods.

Ive known people to bring all sorts of strange stuff back as hand luggage and never been queried though. But as already said, if it has value and is coming into the UK then duty and VAT is liable.

There is a select few range of goods that arent liable for duty, but they are mostly weird stuff. Again you would have to prove your goods fall into these criteria
Stuff like antiques, some medical gear, stuff purely for testing/destruction, temp imports like repairs etc.

You'll need the paperwork to support it though. I got a list of them one time from customs