Hi,
If you are resident in the USA and commence receipt of UK based pensions, you need to download and print off the form DT individual UK/USA (UK-USA (SI 2002 number 2848) (form US-Individual 2002)) which can be found at:
Double Taxation: UK-USA (SI 2002 number 2848) (form US-Individual 2002)
This is not the same as the form 8802 you mention.
On the form, you declare the pensions you have commenced receipt of, you signed and date it and then send it to your local IRS office for validation. The IRS will validate the form, confirming that you are resident in the USA for tax purposes and the pensions are taxable there. The IRS will send the validatef form to HMRC to process.
HMRC can then contact the pension providers and instruct them to refund any tax they have deducted.
Thank you.