Looks like you're a B&W man. It's a very good idea to keep the speakers matched across the front, but, personally, I don't think it's that important to limit yourself to one brand for surrounds. That is unless you listen to a lot of surround music and you are worried about timbre matching (still, most vocals are going to be in front). I have very different surround speakers compared to the rest of my setup and I listen to multichannel music - my setup does just fine. Balancing the speaker levels and dealing with the room acoustics is the key.
Surrounds don't do nearly the work that the fronts do, so I say you shop around for other brands that may work better in your room. Don't limit yourself. Go test drive some other speakers and you might find a pair of surrounds that fit much better.