Currently I actually have enough UK-sourced employment income that I do not need to make voluntary contributions. However, from next year I will be applying to make class 2 contributions as I meet the requirements.
However, if I at some point take a career break or even retire early - can I continue making class 2 contributions? The website is a bit unclear. The "or you worked while you were abroad" suggests that I still could, but it might mean you were working during the year you are trying to make a contribution for. If you are unemployed and living abroad - can I make class 2 contributions? Then if I subsequently go back to work, could I resume class 2 contributions?

(My guess is that in practice most people simply keep paying class 2 and never even talk to HMRC about it if they stop working, But trying to establish what the rules are.)

https://www.gov.uk/voluntary-nationa...-contributions

You need earned income in order to make NI contributions (same way you can can only make SS contributions with earned income). So if you are fully retired for a whole year, then no. But if working part time or resuming work in a subsequent year, then yes.

On a related note, if you decide to start receiving your state pension then decide to do some work, I'm not sure what happens to your pension amount. You probably know on the SS side if you take SS before your full retirement age then go back to work your benefits are reduced after your income exceeds a threshold. Not sure if same applies to UK state pension.

They told me when I called that you have to "have been doing work" during the year you want to make contributions for. When I asked if you had to be employed or did self employment count, he just reiterated "you have to have been doing work". I think they must be able to confirm with the US SSA, because the only year they wouldn't let me top up (initially) was one year when I showed a zero profit and zero income. Although in 2024 they told me I was eligible to pay for 2 years and I asked them to apply it to 2023/24 and the previously missing year - which they did.

Interesting, seems a bit loose.
Would being self-employed as my own investment manager or working 1 week as an uber driver count I wonder.

Well, do you pay yourself and pay FICA taxes on that income when you file your tax return? If so, you have earned income. If you don't pay FICA taxes, you don't have earned income.
Is there a minimum threshold for earnings in order to make a year's worth of NI contributions?

Is payment of FICA the definition of working for HMRC?

You only pay FICA taxes on earned income which is wages/salary/self employment income.
Income from other sources is passive or unearned, therefore not from work.
FICA taxes go into SS.
Same thing in the UK. NI contributions are paid from earned income only. So if you did not pay FICA taxes I don't think HMRC would consider you to be working.
How many years NI contributions do you have? What's your estimated pension worth right now? Is it worth building up more years?

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I qualified for Class 2 as self employed even without payment for the tax year. I deferred payment into a different NI (and US tax year) so no FICA contributions. Did this a couple of different times. No questions or inquiries ever made so either they only check first time you apply and don’t check for ongoing contributions, check randomly or they don’t care at all. Personally, I think go on what you tell them and if they validate with FICA contributions it is on a random basis or more likely because something unusual prompted them to do so (like an application to start or restart contributions) but I don’t know that for a fact.

Here

"But you may choose to pay Class 2 contributions if......... This applies if you’re any of the following:

• an examiner,
• a minister of religion
• a person who makes investments - but not as a business and without getting a fee or commission"

https://www.gov.uk/pay-class-2-national-insurance