if i buy an iphone from apple UK, can i take advantage of monthly installments?

You would need to go to the Apple website to check. I've tried checking here, https://www.apple.com/uk/shop/buy-iphone/iphone-11-pro/5.8-inch-display-64gb-space-grey and it seems it should be an option, if you have the Barkclay's card.

The only limitation I see is that it has to be connected to the four major US carriers (AT&T, Sprint, T-Mobile, or Verizon). While the pages does not specifically say you can't I think it is implied that it works only in USA.

Note also that you can only purchase from your country's Apple Store, there are no international shipments. The phone's warranty is not international, either.

Were you a member of an educational institution, there may be an educational discount available.