Despite some widespread delays from manufacture, UK availability of the Lumia 950 is gradually increasing. The latest is MVNO, giffgaff, where you can buy the phone on monthly installments without committing to a cellular plan.
And as forums member daniel_sherlock points out, that means you can buy one on a monthly plan and use it elsewhere, since giffgaff sells unlocked phones.
Full story from the WindowsCentral blog...