How exactly would customs know that any unpacked goods you have with you were purchased there?
Even if you threw the packaging away, most people won't spend over £500 on something and just throw the receipt away.
My family got stopped by customs some years ago on return from Florida. They went through all our cases and checked every one of the receipts my Dad had to ensure it wasn't over the threshold.
So while you may likely not get searched, its naive to think its impossible, as has already been pointed out, customs aren't stupid.
Also why do we always get these threads from people going to the US? Surely if there are items you need, you already know what you want. Just enjoy your holiday, its not some kind of cheap Mecca and its been a long time since the pound offered significantly higher spending power.
have you ever been? you can get good brand clothes far far cheaper than you can over here most of the well known stores have huge discounts for holiday goers as well! usually you can find them around the travel information areas of mallsAlso why do we always get these threads from people going to the US? Surely if there are items you need, you already know what you want. Just enjoy your holiday, its not some kind of cheap Mecca and its been a long time since the pound offered significantly higher spending power.
I don't understand how this works, say if I'm given a camera from the UK and take it on holiday with me. On return they can charge me customs on it, if I can't provide a receipt? (as it's a gift)
have you ever been? you can get good brand clothes far far cheaper than you can over here most of the well known stores have huge discounts for holiday goers as well! usually you can find them around the travel information areas of malls
As someone else has said a lot depends on where you are going in the states, Minnesota for example has no tax on clothing sales so clothes are very cheap.
everything
Petrol ?

Other goods including perfume and souvenirs
You can bring in other goods worth up to £390 without having to pay tax and/or duty.
If you arrive by private plane or private boat for pleasure purposes, you can only bring in other goods worth up to £270 tax and duty free.
If you bring in any single item worth more than your allowance, you must pay duty and/or tax on the full item value, not just the value above the allowance. You also cannot group individual allowances together to bring in an item worth more than the limit.

That's not what I said.
If you go to the US for example and spend $1200 on an Apple Laptop, $800 on a camera, $1500 on a watch etc....
Its highly unlikely that you are going to throw those receipts away because of needing them for warranty, tax, insurance and credit card protection reasons. Many big ticket items will also have the serial number printed on the receipt, Apple certainly do.
So if you get stopped and searched, they can easily check things like this. Which as I said above, is exactly what they did to my family. We were young and had largely bought lots of Disney stuff and clothing and they were making sure that they didn't exceed the exemption.
So if you sent all of your receipts home in the post, they have no proof that you bought the stuff in the USA and can't charge you?
I don't know the details of how they work.
Ultimately if you had a suitcase full of brand new looking goods, an amount to clearly take you over the exemption then chances are you are going to be questioned on them. How they go about charging you for breaching that and forming evidence without receipts and stuff I don't know.
Chances are you are unlikely to be searched anyway, but it can happen.