London has paper transactions, but is also a delivery market, where large quantities of real gold change hands. The gold may not actually leave the vaulting system: the transfer can often be accomplished with a warehouse receipt.
However, since November last year, clients of the COMEX (often acting through JPMorgan bullion bank) have been standing for delivery, e.g. 62 tonnes in the 2 weeks after Thanksgiving 2024:
https://www.goldmoney.com/research/massive-comex-deliveries-...
Now JPMorgan is standing for delivery of 93 tonnes in February 2025 (3m troy oz, $4bn):
https://finance.yahoo.com/news/jpmorgan-plans-4-billion-gold...
COMEX has had to scramble to find the physical gold to deliver. The US price went up, creating a spread with London and an arbitrage opportunity. Physical deliveries from the LBMA vaults in London were flown to the COMEX vaulting system in and around New York.
If the deliveries cannot be made in New York, there is a rather murky process called Exchange For Physical (EFP), where the client is given a paper promise for gold delivery in London. Not sure if there is client discretion, or if they can be forced to accept EFP by the COMEX T&Cs.
I think that COMEX does have the right to cash settle delivery requests, but that would be seen as a soft default, and seriously damage its reputation as a physical market grounded in delivery.