Date I last filed UK tax return (20+ years ago)

I left the UK in 2003 and now wish to start withdrawing funds from a private UK pension. Part A of the US double taxation form asks for the date I last filed a UK tax return. I believe I filed in March 2003, but I no longer have a record of the exact date. As it was more than 20 years ago, would a "best guess" be acceptable, or do I need to be precise? (I did try to set up an account on the GOV.UK mobile app to see if I could access my tax history, but my UK driving license and credit history were too old for ID verification.)

Hi,

You can give a best guess estimate.

Thank you.