Would it not be cheaper to sell your existing IPhone and pool the money from that and what you have reserved for the spare iPhone 13 towards importing a Hong Kong iPhone Model Axxx with dual discrete nano-SIM slots?
(You should also be able to pay in $HKD if you load a balance to a Wise debit card to avoid transfer fees from $USD - presumably you use this loaded with £GBP when you are at Glastonbury?)
Using Dual SIM with two nano-SIM cards
Published Date: January 10, 2023 https://support.apple.com/en-hk/HT209086
so long as it supports the cellular bands you require in your home country (California, USA?):
5G and LTE. Find the iPhone that’s right for your country or region. https://www.apple.com/hk/en/iphone/cellular/
Identify your iPhone model
Published Date: June 06, 2023 https://support.apple.com/en-hk/HT201296
I personally plan to buy the final 'Max' or rumo[u]red 'Ultra' HK iPhone that uses a Qualcomm modem, before the switch to an Apple-designed one (similarly, I plan to soon buy a refurbished base-level Intel-base Mac Pro 2019 now that it has been discontinued in favour of the Apple Silicon-based Mac Pro 2023).
It is my understanding that it should still be possible to register for AppleCare+ - which provides global coverage - from the option within your HK iPhone, up to 90 days after setup / initiali[s/z]ation of the phone landing on US soil, using your regular US Apple ID with payment in $USD.
