That's a big enough number that the likely result is Apple would pull out of the UK altogether.

Does it matter? Apple paid around 300 million pounds of corporation tax in 2024. If the UK poses a landmark penalty of say 50 or 100 billion pounds, thats 167x or 333x the annual corporation tax paid by Apple in the UK.

What on earth makes you think that Apple would pay if they leave the UK?

I'm assuming if they pulled out they wouldn't pay.

Then good riddance.

That would be an amazing opportunity for competitors who chose to continue operating in the UK.

That's not what would happen. People will just import the iPhones from abroad.