Backdated Pension

Two questions today!

I'm a UK/US citizen - grew up mostly in UK and have been living back in the UK for a number of years.
I worked for the NHS many moons ago and built up a pension.  I am now retiring.
My normal pension age would be 60 but I forgot to contact the NHS until I was >63 so I have received a  "back dated" sum, which has been
taxed in the UK.

I presume that I need to report this back dated sum as earned income, but I should not have to pay US taxes on it?

I will also be receiving a (UK) tax free lump sum, which I understand that I WILL have to pay US tax upon. Whats the best way of reporting this? Up to now I have been using Form 2525 (Foreign Earned Income) but should I start to use Form 1116 as pension payment is not strictly earned income?

I am not a tax pro but am a UK/US citizen living in the UK receiving both private pensions and the UK OAP.

Since pension income is not earned income it cannot be excluded as a FEIE.  Need to use foreign tax credits to reduce US taxes.

For me OAP is only taxable in the UK and so will SS when I start drawing it.

I would think an NHS pension is classed as a government pension since it is paid out of tax revenue so I would say that it is taxable only in the UK as per article 19 of the treaty. I know someone who is receiving a California government pension that is only taxed in the US following that same rule even though he lives in England.

See the technical explanation on article 19 below

https://www.treasury.gov/resource-center/tax-policy/treaties/Documents/teus-uk.pdf

Actual treaty is here

https://www.treasury.gov/resource-center/tax-policy/treaties/Documents/uktreaty.pdf

Lump sums paid out of UK pensions are subject to much debate here and may or may not be taxable in the US.

The 25% tax free element is contentious. Some say that as the 'pot' is taxable and only HMRC 'allow' 25% tax free, then this amount is taxable by the IRS. On the flip a Roth IRA is tax free totally and accepted so by HMRC.

The only option that provides certainty is to not take the 25% tax free option.