Unlicking a phone when it is impossible to meet the required conditions

I visited the USA , Park City, to be precise from the UK where I reside being a Bristish citizen, for 10 days starting on the 30th March. Unfortunately, I inadvertently  left my UK cell phone on the aircraft when I flew from London to San Francisco but I didn't realise this until I arrived in Park City. I wanted therefore to be able to obtain an ixexpensive cell phone for use while in the USA to provide emergency contact if need be with my family. I went to the AT&T shop at Kimball Junction in Park City who directed me to a local store where I could buy an inexpensive phone and there  I bought a second hand iphone 4. I returned to the AT&T store who provided me with a 10 day sim card with international access for $25. I subsequently topped up my credit with a $10 gophone payment. This all worked perfectly fine and served me well while I was in the USA but on returning to the UK I could  naturally get no service from AT&T and I found my phone is now locked to AT&T.

I've tried chats, speaking directly to the gophone support line, calling the AT&T shop that originally sold me the 10 day sim card  and applying through the online portal to unlock my phone but all to no avail with the reason for rejection of my request to unlock the phone being that this cannot be done until I have had 6 months paid for usage which without any service n the UK I obviously cannot do. I was not advised when  buying the 10 day sim card service form AT&T that this would effectively make the phone I had bought unusable by me after the 10 day sim card had expired.

I can see why service providers' rules  require a minimum period of service when they have provided a subsided phone as part of the deal but this is obviously not the case here. How do I get my phone unlocked and get to use it?