You could double that number to account for the "fraud and error" spend at DWP: I would presume that further spending in this regard is dedicated to other departments.
Sure, or you could halve it since much of it's admin for pensions, and stuff like disability allowance and housing benefit you don't want to eliminate unless you're cool with UBI leaving people immobile or homeless, and you can't do payments to 36m working age people with no admin. Either way, it's still an order of magnitude or two less than UBI costs.
Sure, you can fire a few low paid Jobcentre employees if you're convinced retaining that sort of admin offers no actual benefit to anyone attending, but it turns out that paying 8 million people who are neither in employment nor willing/able to ask for appointments at Jobcentres a livable income isn't cheaper
