It's a tricky one. As far as HMRC are concerned, you should declare it, and pay duty at the correct rate, plus import VAT! They consider not paying it as smuggling
Now, I bought two of my Breitlings duty & VAT free in Jersey CI, (not at the same time I might add) and brought them back in without declaring them ( whoooo ... I'm a smuggler now
) and wore them on the plane and through the airport. I made sure i didn't attract any attention with lots of duty free bags, and we had fairly light luggage, so I figured I doubt they'd ask, and I just walked out via the green channel (though there wasn't a customs official in sight in either the red or green channels either time I might add). I did however post the boxes, manuals and receipts back though, but it was probably easier from Jersey than you'd find from Thailand.I suppose your alternative to avoid fairly punitive taxes on a £4k purchase is to forego the box which is bulky and probably too easy for an eagle-eyed tax spook to spot, and to hide the receipt and manuals in other holiday documentation. Unless you plan to bring back twice your tobacco and booze allowance, and intend to smuggle in prohibited substances, in which case you deserve to get pulled out of the line for a 'thorough examination'
and as long as you don't look shifty, weighted down with lots of luggage that 'could' hide smuggled goods, you'll probably be ok. Just look like normal Brit holidaymakers/honeymooners wanting to get home after a long flight..... Of course, I'm certainly not endorsing depriving the UK Government of your hard-earned cash, as we really don't pay enough in taxes here do we Good luck with the wedding, and good luck with the purchase.