I’m not sure if you are asking if you should intentionally not declare but that kind of conversation doesn’t tend to fly well around here, to be honest.
For some people, Europe is cheaper. If it is cheaper in the UK, I recommend buying it there and just enjoying your holiday. If it is cheaper in the UK but unavailable, well then I would view the duty etc as a “opportunity tax”. I paid it for the opportunity to get the item I really wanted and love.
Also keep in mind that border officers are receiving more sophisticated methods of tracking purchases. Knowing your obligations with respect to duty is expected. Should you find yourself the subject of an investigation, it could potentially really impact future travel among other things. To me, it would never be worth it.