Oh really ARM is not selling/operating in Europe? Brexit or not it doesn't change a thing. Last big merger (Qualcom and NXP) was blocked by China. Even if EU don't do it China is likely going to mess Up with this one again.
This isn't about Brexit being a distraction, it's about their mandate and optics.
Until the 31st of December the DG COMP actually needs not only to show that there is a sufficient reason to investigate the deal, but that the CMA ruling wasn't in line with the TFEU.