None of it is taxable on you 1040 and you include 90% of it on your UK taxes...I don't know if you can claim a credit on your US taxes for the UK tax you pay on SS.....I imagine not...maybe the better informed can answer that.
As you well know, one of the problems F4m will have if he decides to file tax without the aid of an accountant is finding any instructions/directions within the IRS publications/information explaining where to declare
foreign pensions and how to calculate/ report them. You sense they should be reported, but information from the IRS regarding
foreign pensions is like finding hen's teeth in the dust.
I've always taken the stance that by invoking the treaty (not declaring SS payments as taxable by the IRS) I also can not use the UK tax paid on them when completing form 1116 to calculate FTC's. I proportionally reduce UK tax paid by the amount of the US SS to all other like income. But, I can find no written confirmation, anywhere, especially from the IRS, which provides any insight. It just 'feels' right.
Guya's comment presents an intriguing consideration, and another conundrum. I've never seen someone seriously recommend to opt out of the treaty, but what happens in the confines of an accountants office, I imagine, would be mystical to most of us, but done with a purpose. If you opt out of the treaty for the US (declare US SS on 1040 in line 20b) and pay the additional tax, can you opt out of the treaty on the UK side? Will HMRC allow you to opt out, even though you receive the funds in the UK? If not, then you're double taxed to begin with, but DO use UK tax paid on them to calculate FTC's on 1116. It will have the same NET effect on the results for 1040, so why do it. Well, one reason could be to gain additional carryover/carryback FTC's. (I've thought of doing this in the past.) I suppose it works in a year you need additional FTC's. But again, what is your goal? Simplification, or going for broke to reduce any possibility of tax both now or in the future?
Rabbit hole, indeed.