Comment 1) why go to the US (Stewmac) for a nut when there are so many good parts suppliers in the good old UK? Comment 2) The guys advising you to get it done professionally are absolutely right.
Comment 3) Start supporting the UK guitar build/repair/parts industry so that it will still be here when you really need it, If you get copped for VAT and import duty it will cost you about 3 times as much as buying the part here.
Lecture over.
I kind of agree with the sentiment of what Bob is saying.
Whilst i too have bought stuff from USA and have bought a ton of stuff from Stewmac - there is an important reason to buy through the UK dealers or certainly from the small specialist repairman.
There will be various times when you need that specialist to do some work for you that you don't want to or can't do yourself- it may be a set-up, or a refret or even a complete rebuild or building a custom guitar.
You will still want to find him trading as a repairer and not have given up through lack of money and gone off to become a postman or working in an office or supermarket, or wherever will pay enough to keep a roof over his head.
Putting trade his way and buying bits and pieces from him helps keep him employed in the job where his skills are most useful and appreciated.
I'm not suggesting that you pay way over the odds for big stuff but certainly over small stuff where you would be penny pinching to have to import it.
Also his experience means that you end up getting the right part first time.
I have had many guys turn up with parts they have bought to fit to their guitars - only to have to tell them that they have gotten the wrong part, or as has happened a couple of times that the part is faulty.
If i had been supplying the part and it was faulty then I would replace it and get the job done right, but if they bought it elsewhere then they would have to seek re-dress with the place they bought it and that can become a drawn out situation.
I've sometimes ended up selling them the part that they should have bought, and getting the job done, leaving them with the wrong or faulty part to get returned.
So basically look after those that you would wish to see still in business in years to come - or lose them and regret it later