I agree. The rear speakers are for ambience, so timbre matching isn't as important. You could replace the centre speaker to match your Mission mains, but if the Tannoy centre already blends with the Mission, then why bother?

LCR speakers of the same brand doesn't automatically mean they'll sound better. My centre speaker is of a different brand to my mains, but it matches much better (in my opinion) with my mains than the mains' own centre speaker.