The initial fingerprint and facial scan is only done once, then it goes into the EES database (and lasts three years I believe.)

It's the initial scan that is likely to cause queues at airport arrivals once things ramp up.

For Eurostar those machines are not at the border, they're before check-in, there's a booth opposite business check-in with several kiosks.

Subsequent checks are to verify against the database to see if person+passport match. That isn't done at the scanning kiosks, but at the border crossing point, either with EES self-service kiosks/egates or a staffed border check, still have to get the passport stamped until things are fully live in April..

Holding a business ticket, staff told me I'd be able to do the initial scan whenever I turn up.

That avoids getting stuck in a potentially huge queue on arrival at an airport.