If you invoke the US-UK tax treaty IRAs should be subject to taxation only in your country of residence, that is the UK.
In practice many financial companies in the US (Fidelity for example) will ignore the tax treaty, even if you invoke it and will withhold a mandatory 30% on any IRA distribution of any size to a foreign address. Fidelity also will not allow regular periodic distributions annually to a foreign address, so you have to apply each year for any distribution you want. Other financial companies will be more flexible so you should put effort into finding out how the company(s) you have your IRA with deal with foreign distributions, and change company if this is not satisfactory.
You will then have to reclaim all this withholding back by filling in and submitting a 1040NR to the IRS each year you take a distribution, and claim a 0% tax rate by invoking the US-UK tax treaty on this form.
You will also have to declare the income to HMRC and it will be taxable, so at some point in the year you may have been "taxed" twice, but the US part is reclaimable.
