Unless you are lucky and see a 2nd hand vox.
I would go into Richers and have a listen to a few speakers. Can you demo the Kefs?
I have no idea what other make of centre speaker would most closely match the Regas.
Try to listen to the Kefs and then have a demo of say the Q Accoustics 2000i system.
See what you think. If you like the Q Accoustics a lot more, could you get just the centre speaker, see how it matches with your Regas, and if it does not work, save up for the 2020i (or 2010i) for front right & left.
Just go and have a listen, it might clear your mind a bit.