The problem is not that he is in the UK, and you would have had the same problem of Paypal hanging onto your money with a US Buyer.
Play their game, and invoice 5 bolts. Once you have met their history requirement, payment is immediate - that is not to say that they still are without issue
A US only policy leaves ethical purchasers with no ability to buy in the US
There is no interoperability between US and Canadian banks, so things like Zelle in the US, or Interac in Canada, will not work for payment by an individual. A cross border cheque is a problem unless one has a USD chequing account, so that leaves trekking to the bank for an expensive wire or bank draft (similar to a US cashiers cheque). As much as I despise Paypal, there are few alternatives for international transactions.
One of the few alternatives are non-bank internet transfers. One is Wise
www.wise.com and I have been using them for several years to send myself money internationally every month, but also for international purchases. I prefer people to pay me using Wise - low fee, fair exchange rate, easy, safe, and far cheaper than the banks.
Some Buyers cannot be bothered to set it up, and prefer the ease of ubiquitous Paypal
The problem seems to be that Paypal wants to hang onto your money, and will not release YOUR money until X number of transactions. Just wait until some wanker claims that he did not receive the part, or claims any problem whatsoever, then see how you like Paypal..........Besides their high handed attitude, their fee, rip off exchange rate, imposed currency exchange from USD to (in my case) CAD, and most annoyingly the international transaction fee, are icing on the shit cake.