A complicated situation, but it can be eased somewhat with double taxes avoided. First, forget about Form 2555 since it only pertains to earned income. None of your mother’s income is ‘earned’. (A bit of a misnomer since I’m sure your parents worked hard to accumulate it.)
I assume your mother is still living in the UK, not the US (since she pays UK tax). A minor point, but is the ‘UK pension’ a works pension, or UK State Old Age pension, or both? Is the survivor’s pension truly from a ‘company’ (like GM, or Bank of America)? Is the source of the US pension truly from within the US, and not a UK pension paid by a US company? Based on the fact that she pays taxes in the UK, I assume her income is over £9,940 (if she is over 65, or £10,090 if over 75) and due to the US Social Security, IRA, the US pension, and possibly (if it’s her only UK sourced pension) the UK State pension, she completes the Self Assessment Form for HMRC. I assume the IRA is a periodic payment, as is the US company pension. I assume she does not receive UK pension credits.
Fortunately, you have two allies, the US/UK Tax Treaty, and Form 1116 (Foreign Tax Credits). Unfortunately, as you’ve discovered, Form 1116 requires a great deal of understanding, and invoking the treaty in either the US or UK can be complicated. As you mention, the treaty will allow for tax to be collected on US Social Security in the UK only. But you will need to file Form 8833 to invoke the treaty for the Social Security, and possibly, to ‘resource’ the IRA to UK income (but it still has to be declared in the US, hence another reason for Form 1116). A credit for tax paid to one country can be accomplished either on Form 1116 (US), or on the HMRC Self Assessment form (UK).
As you can tell, this all becomes a bit of a challenge, and we haven’t yet discussed the company survivor’s pension. For that reason, I would urge you to seek competent assistance. A competent UK accountant may be able to handle the UK side but I, personally, would select an advisor in the US very carefully. They should have a proven record of dealing with the US/UK treaty. My own preference would be to seek an advisor based in the UK who understands the HMRC implications, as well as the US tax code. Allow them to do the returns for a year or two, and after that, you will have a template to use if attempting them on your own. If you prefer to go ahead and attempt them on your own, then you should be aware that each pension has its own unique situation. For example, a UK State pension is taxed in both the UK and the US, but for the US, you use Form 1116 for a credit on the UK taxes paid.
For a person on a fixed income, it’s a bit uncomfortable paying the amounts of monies involved, but I’m afraid that’s solely due to the US Citizenship. If seeking advice in the US from the normal sources (H & R Block, tax help at senior citizen centres, etc.), you may find you know more about the issue than they do.
As always, I am not a professional tax advisor, and my comments may well be incorrect.