OK I'll pull it out completely; I never do very well measuring while its in the cabinet...
And I don't think the 70x100 is a big deal. Anyone ever tell you your a bit picky...

Yep they are 27" x 40" give or take an 1/8". The in cabinet error is 1/2"..

I would think that if they were manufactured in the UK they would basic one-sheet measurements to fit the backlit frames; I would say we are ok here. I am saying British posters and the standard 1-sheets are 27x40. Maybe the prints are 70cm x 100cm?