Remittance basis and National Insurance threshold

I had paid employment in the UK until the end of June 2023. I then was paid a lump sum fellowship amount from an overseas academic institution to an overseas bank account, from which tax has not been taken abroad, but which I have not remitted to the UK. I stayed in the UK until mid-October. I qualify for split year treatment from the time I left in mid-October, so my income from now on is irrelevant. I was a UK tax resident prior to mid-October, but not domiciled in the UK. I am deciding whether to choose the remittance basis or not for the 2023-2024 UK tax year. I understand that if I choose the remittance basis I will lose my personal allowances (ITA07/s35) and AEA (TCGA92/s3(1A)). What I am not sure of is whether it only applies to Income Tax proper, or also to the cutoffs below which I pay or do not pay National Insurance. For example, let's say my foreign source income is equal to the Personal Allowance (£12,570). If the loss of personal allowances also impacts National Insurance contributions such that I would pay NI from £0/month instead of from £1,048 on my UK source income, then there would be nothing either gained or lost from choosing the remittance basis. If, however, the loss of personal allowances only applies to Income Tax, then claiming the remittance basis would save me 12% (£1508.40) on the £12,570. So does the remittance basis have any impact on National Insurance contribution thresholds on my UK source income?

Thinking again, and using the online tools, I think the payment was made in such a way, that I may count as self-employed in the UK. If so then I think I need to pay class 2 contributions (£3.45/week) but not class 4 contributions because I am not making more than £12570 in the tax year from self employed income - even though I also have income from other employment (which is total is more than £12570). Does this sound right?

Hi Cam W

There is a table of rates for national insurance here Rates and allowances: National Insurance contributions.  
For 22/23, the lower profit limit was £11908 and 23/24 £12570.  
Any net profit above the thresholds, will pay class 4 NIC at 8.73% for 22/23 and 9% for 23/24.  
Class 2 NIC is voluntary up to net profit of £6725, after which it is complusory and is £3.15 per week for 22/23 and £3.45 for 23/24.

Thank you

Hi Cam W
Unfortunately, without seeing your personal records I am unable to advise you.
Your first step would be to contact our taxes helpline for advice. You can contact them on 0300 200 3300 (+44 13 5535 9022 if calling from abroad)
lines are open from 8am to 6pm, Monday to Friday.
Following their advice, if you still have a query with National Insurance contributions, please contact our National Insurance contributions helpline
on 0300 200 3500 (+44 191 203 7010) lines are also open from 8am to 6pm, Monday to Friday.
Thank you.