Apple, as the innovator of the smartphone, had the lucky fortune to be the only supplier in a very good stock position for years in the early era of the carrier arms race. In the vast majority of countries there were competing carriers looking to lock up customers and discounting the phones as a result.
As for the UK, if the iPhone's weren't subsidized then you should have nothing to complain about now except for scarcity of inventory and a terrible exchange rate with the US, you are already used to paying more for Apple products than others do.
For the third time, Apple doesn't want to shed customers. But it's out of their control. Here in the US, their biggest market, where I used to get a $0 iPhone 5C for my daughter and a $99 iPhone 6 for myself on a two year contract, I now need to spend $349 for my daughter to get into an SE and $1250 to get into an X. That's not because of anything Apple has done, it's the carriers ending the gravy train. I'm sure Apple would love the carriers to offer ridiculous subsidies again, hell they'd ship 3x more product that way.