If you go through the "Items to declare" lane at an airport, presumably you would declare any items on which you think that duty/UK VAT might be payable. It's not the Customs officer's task to find those items. Compare that with the mail service, which carries many items that have no duty/tax liability. Do you think Customs have the resource to inspect every single package to see what it contains? I don't know what percentage of items are inspected, but I would expect it is rather low. There should of course be a Customs Declaration on the outside of the package.
Larger items, such as vehicles, are often imported UK taxes paid.
Larger items, such as vehicles, are often imported UK taxes paid.