This might be helpful:
https://wise.com/gb/import-duty/from-usa
Basically, if it's declared as a gift and coming from the USA to the UK, then if the value is under £135 you shouldn't be charged any import duty, although you may still be liable for VAT (and hence also quite likely some form of handling fee by whichever carrier is dealing with the delivery).
Of course, if the sender is willing to under-declare the true value, then the chances are you'll be charged no import duty or VAT. Be warned however that Border Force / HMRC are legally allowed to open any delivery you receive and if they think it's been under-declared you may well be asked to provide proof of the value, and if it's greater than the declared value, you'll be charged whatever fees are due. If it is under-declared, unless they can prove that you asked the sender to under-declare the value, then you won't get in trouble and neither will the sender, but as I say, you may well still have to pay fees. More importantly though, it will also mean that your delivery will have been opened and fingered and then re-packed by someone who, at best, probably won't appreciate what constitutes damage to a rare vintage item, and at worst simply won't give a ****, meaning your item may well get badly damaged, and getting compensation out of them, so I am told, can be extremely difficult.
Having said that, I've received numerous deliveries from abroad over the years from the USA and elsewhere where the value has been underdeclared on both items marked as goods and also gifts. I've never requested that the senders do this, but it's great when they do as you save yourself a fortune in import duty and VAT. So far I've been lucky and only a few of these deliveries have ever been opened by Border Force (although my last one a couple of weeks ago was), however they obviously haven't been collectors / knew the true value as they didn't query the declared value. I have however had a delivery some years ago that was sent as a gift (again, not at my request), where I was asked for proof of value, and I ended up having to pay charges, so it can happen.
Hope that helps.