Originally Posted by
WilcoRoger
To exit the UK, you first have to get into the UK
Also, I wouldn't trust the gate agent knowing what an electronic ID is, how it looks like and if it's a valid travel document to Finland.
Haha yes, but OP asked about flying UK -> HEL. You could for example fly to the UK and loose uour passport, or give it to an embassy for a visa application (no idea how you would retrieve it afterwards though).
I honestly don’t know what would happen at the gate. In my experience, some agents like to make up their own rules. I’d give it a less than 50% chance.