Yes, both companies have offices around the globe, with corporate entities all over the place. As those have to operate within the laws of the countries they're incorporated in, if they get blocked by some local watchdog, the acquisition wouldn't be able to be completed in that country.

More so, the merger is also discussed in the EU in general, which is a bigger market than just the UK alone, and if the acquisition gets blocked in the UK, EU watchdog will surely use that block as prior material for doing a EU-wide block.

Hence Microsoft is lobbying both the UK itself and Europe wide for making the acquisition go through.

Of course, even if the acquisition gets blocked everywhere but in the US, the US counter-part can still be acquired, but not sure how much sense that would make, they'll probably end up not going through with it at all in that case.