Iphones are vastly different to android. I'll explain how android updates work then.
Google creates a new version of android, the phone manufacturers modify the update to work on their devices, then the cell carrier approves the update once they know it will work on their network. Different phones are made doe different regions depending on laws and signal requirements. Eg, american iphones have mmwave antennas, no other country iPhone does.
With all this in mind, you have an american phone wanting american updates, but with a British cell carrier. The cell carrier won't jave american updates on the approve list as an american phone is supposed to be in America.
For iphones, all iphones are the same and get the same updates, so there's no cell carrier involvement.
In modern android, there are solutions for this update problem. There are 2 update systems, system update and Google play system update. The Google one works on every phone regardless of country, manufacturers or cell carrier. Google makes the update, it installs. Plus now a lot of system services get updates through the play store.
So tldr, you have a phone in a country it wasn't designed for. Your cell carrier isn't expecting american updates so doesn't have american updates on the known working approved list.
All android devices work like this
