Hello, I am in a similar situation, naturalised USC / born UKC, planning my move from USA to UK.
Here is how I would answer the OP's question.
IMO
If you are UK resident & domiciled then you are liable for UK tax on your world-wide taxable income. If you have income then it is likely that you should be filing a UK return. If you haven't done so then I suggest you visit your local HMRC tax office and discuss with them whether or not you should be filing a return.
You don't say what what type of US Retirement e.g. Roth IRA, Traditional IRA, 401k etc ?
If you withdrew from a Traditional IRA or 401k then, as everyone else that has one of these, you are liable for US income tax on it - and also UK income tax by falling under the HMRC tax laws. However, having paid US Tax on the withdrawn income, when you make your UK tax Return you would enter the withdrawn amount + the foriegn (US) tax that you have already paid andd this will likely be credited to you , reducing your potential UK tax amount.
If you withdrew from a Roth IRA then this should have been free of all income tax in the USA and because of the Tax Treaty also free of all tax in the UK and therefore would not be reported as UK income because it is not as taxable event. If you did have US Tax withheld on a Roth because you have a non-USA address then you can claim that tax back next time you file your US tax, as you should do as a USC.
Good luck.