I was trying to figure this out too, other reputable sources cite 600k as well.
There is some number of passengers who have not yet left on their trip, who are part of that number.
There is some number of passengers who have not yet left on their trip, who are part of that number.
It's 150k UK vs. 600k total holidaymakers.
Booking holidays often means scheduling time off work, around children's school holidays, when prices are reasonable, etc. Not always a trivial task.
Having those plans abruptly cancelled in this manner where they need to not only get a refund for any money paid, but to also try and make alternative arrangements on their own and cover any difference in cost out of their own pocket is effectively leaving them stranded in a sense.