Hi,
I am a UK citizen who also has a US pension so offer advice from my experience.
--- As is said, under the tax treaty (Article 17(1)(a)), as you are resident and domiciled in the UK the UK has the primary right to tax your pension. HMRC has recently stated the following that I hope you find useful.
G
Periodic Payments
Periodic, frequent, payments or withdrawals, that is weekly, monthly, annually and so on, then those payments would have been taxable within the UK and ‘maybe’ exempt from US tax. This is in accordance with Article 17(1)(a) of the DTA which, again from the perspective of a UK resident, states:
‘Pensions and other similar remuneration beneficially owned by a resident of a Contracting State [UK]shall be taxable only in that State [UK].’
As a result, these periodic payments are fully taxable in the UK and should be declared to HMRC on the foreign income pages (SA106) of a Self Aassessment return. If US tax has also been paid on those payments, then it is important to note that no UK tax relief can be claimed to offset that US tax charge against any UK tax due. Instead, you must approach the US Internal Revenue Service (IRS) to claim US tax relief and the type of US tax relief available will differ depending on whether or not you are a US citizen.
If you are not a US citizen, then periodic pension payments will be fully exempt from US tax and you should claim a full repayment from the US IRS. However, if you are a US Citizen, you will only be permitted to claim the US version of FTCR, which will offset the UK tax paid against a US tax charge. This is because the US taxes its citizens worldwide income, regardless of where they are resident. So, if you are a US citizen, then Article 1(4), as outlineD above, would kick in again and, this time, allow the US to tax any periodic payment received, despite Article 17(1) providing the UK with the sole right to tax. Again, Article 1(4) effectively ‘overrides’ Article 17(1), and the consequence is that both the UK and USA can tax any periodic payments received.
In these situations, double taxation will occur since both the UK and the USA can tax the same income. However, this time, it is for the US to eliminate that double taxation, since they are the ones invoking Article 1(4). This is in accordance with Article 24(1)(a) of the DTA, which states:
‘In accordance with the provisions and subject to the limitations of the law of the United States (as it may be amended from time to time without changing the general principle hereof), the United States shall allow to a resident or citizen of the United States as a credit against the United States tax on income:
a. the income tax paid or accrued to the United Kingdom by or on behalf of such citizen or resident.’