Originally Posted by
JALPak
Your assumption is wrong, at least not with 06 or any of the A350-1000 so far.
https://www.aviationwire.jp/archives/309890
They may not need the full 3 weeks but it doesn’t hurt to be conservative and move up the schedule if needed. It’s not JAL’s problem per se if someone intentionally booked an inaugural flight and missed it because the dates are moved up
a single aviation enthusiast passenger wouldn't be entertained to that regards, but given LHR would be JL's first European route served by the new aircraft, there might be some sort of ceremony (maybe a discreet one, certainty not as large scale as the JFK one when the aircraft type was first launched), or a brief speech delivered by a someone from the London/European office
even if there is nothing special happening at the gate at LHR, there can be pre-arranged media trips, invited press representatives or sponsored tickets for influencers (Europe/UK-based ones in particular), which needs to be taken into account
an individual passenger might not matter, but a few aviation bloggers or YouTubers would be pretty powerful