I'm afraid that building a quality system won't be as easy as shopping for price over the internet when trying to integrate an existing set of speakers into the mix. It will take some live auditions, listening to speakers to find what matches the existing pair.
The center channel is the most important to match as closely as you can to your existing speakers and will likely be the most expensive. It's important to match the center and mains in home theater, because as sounds travel across the soundstage through the L-C-R, the tone will change when it crosses the center and over to the opposite side if they are mismatched. For example, people's voices as they walk across the screen might sound quite different as the sound mixes through the center channel to the left or right.
I haven't heard the KEF system you are referring to, but if you want to integrate the Royds, I doubt that a prepackaged solution will create a properly integrated system. I stongly advise that you go out and listen to as many center channels as you can and choose the one that most closely matches the Royds, then choose two or four surrounds from the same line and a good sub.
Really good center channels can be had from a number of manufacturers, ranging from Energy, Axiom, Paradigm, Era, B&W, Focal.JMLabs, AV123, and so many others and each has good choices for surrounds. If you try to describe the sound quality of the Royds, members here can assist in trying to point you in the direction of speakers with similar qualities. The same goes with subwoofers. If you can tell us the size of the room and your expectations, we can guide you towards a sub within the budget that will be far better then your typical theater in a box subwoofer.