Eurail paper pass validation at Heathrow Airport T4

I previously asked a question re this but it’s disappeared.

In a few weeks, I arrive at Heathrow Terminal 4 at night.  No guarantee the flight will be punctual, but I’m hoping to be on an Elizabeth Line train by 2100 hours to connect to my destination beyond the London stations.

Eurailpasses are valid on Elizabeth Line (as it’s a National Rail service) but given there’s no booking office at Heathrow T4, how do I get the paper pass stamped and the validity dates of my consecutive Global Pass entered?

Of course, I can always ask staff if I can travel through to a station with a booking office, but what if the staff are grumpy or officious and won’t let me do this?