UPDATE: Well, here's what happened on my travels from LHR to the EU destination:
Arrived really early at LHR for the morning flight, self checked-in my large bags and handed them over at the bag drop desk. Went through to security carrying the coins in my shoulder rucksack.
Asked the nice security lady for a private screening of my rucksack as it contained high value coins. She had a chat with her supervisor and ushered me into a private room just off the main hall. I put the coin boxes on the desk, she ran a swap over the boxes, tested the swab and wished me a pleaseant joureny.
Walked to the airside Customs office just around the corner from security, picked up the red phone and said I needed to make a cash declaration. With in 15 minutes a couple of customs officers walked by and led me into a private room. I gave them my pre-filled cash declaration forms. They briefly inspected one of the coin tubes from one of the boxes, stamped my cash dec. form and wished me a pleasant journey.
Got on the plane with no questions asked regarding contents of hand luggage, vaccination/test status or registration forms.
Arrived in EU country, got past passport control, picked up large bags and went through "Red Zone" exit. Approached a customs officer saying I had some goods to delare, was ushered into the customs office, they were expecting my arrival as I had emailed them the previous day. Gave them the EU cash declaration forms. They were super friendly and very interested in the coins. Forms were stamped and I was on my way.
So... ...a huge sigh of relief, everything went smoothly. I took way too much paperwork and documentation with me, almost none of it was asked for. 😊