Annoyingly it comes in 10m2 rolls
It's always been in 15sqm rolls for me - but I think that's trade suppliers' sizes (bloody heavy if you're lifting it, so don't plan on transporting all your flooring and underlay in one go!). Maybe they do DIY sizes in 10sqm (ah just checked the website, and that is the case).
Just buy the two rolls and be frugal...start your roll-outs a few inches from the wall...that'll probably save you the half a sqm over the room. A roll of cheap duct-tape helps when rolling it out to keep the ends stuck down, although this stuff does tend to lie flat under its own weight. Keep any off-cuts from around fireplaces etc. They'll come in a handy. A brand new Stanley blade will make your life a lot easier, and as it's a concrete floor, maybe get yourself a pack of five.
Remember to roll out your underlay perpendicular to the direction you're running the floor.
EDIT: crossed posts, sounds like you're sorted.