UK pension lump sum and IRS

Hoping someone can help with a pension related question.
I have an NHS pension and when I reach pensionable age I will take 25% of it as a lump sum and the rest will be a monthly amount.
Having read as much as I can on this site and British ex pats, I am confused about whether the lump sum is taxable by the IRS as it is tax free as far as HMRC are concerned.
From my reading, I understand that it would be taxable if I lived in the US  but I live in the UK and can't find any posts about UK pensions being claimed in the UK.
I have dual UK and US citizenship.
If anyone could help with this or recommend an international tax professional I would be really grateful.
Many thanks in advance.

The taxing rules are as follows-
•   The full terms of the UK US double tax treaty do not apply, as you are a US citizen. This is provided for in Article 1(4) of the treaty, the savings clause.
•   Article 1(5) lists some treaty provisions that still apply. These include Article 17(1).
•   The lump sum commutation is tax exempt in the UK. Therefore Article 17(1)(b) then requires that the US is to respect the UK tax exemption, but only if the recipient is a US resident.
•   As you are UK resident, you are outside the protection of Article 17(1)(b) and so the commutation is US taxable.

It may be a moot point. You will have some basis in the pension under the general rule; should you wish to calculate this. Additionally you will be able to use excess foreign tax credits from up to the previous10 US tax years.

Thank you both very much for your replies.
After reading Article 17 it remains confusing so I will consult a tax expert - too much at stake to get it wrong!