Hi simon gordon,
As a resident in the USA, with a UK pension.
The pension is not taxable in the UK, but will have tax deducted until we process the validated form mentioned below.
You will need to print off the form here:
Double Taxation: UK-USA (SI 2002 number 2848) (form US-Individual 2002)
Complete it, sign and date it and send it to the IRS.
They may charge you a fee for this.
The IRS will validate the form and send it directly to HMRC.
HMRC can then ensure that any tax deducted from your UK pension is repaid to you.
The full amount pension received, will be taxable in the USA, if it is taken "periodically at stated times", eg. monthly.
The 25% rule you mention, only applies to lump sum withdrawals from your pension, which would be taxable in the UK.
We cannot advise you on US state or federal taxes.
You will need to seek guidance in this area elsewhere.
Thank you.