Try Butterfields in Luton.

http://www.butterfieldnatstone.co.uk/

If that's the place I remember they have a big yard full of raw stone slabs for worktops and some fancy machines for cutting out holes for sinks and polishing etc I saw it while visiting their brick library. They might charge you "corkage" if you bring your own stone! One problem might be ..what happens if it breakes? Normally if they supply the stone and if it breakes while they are cutting it you don't pay.

bloody hell something i realy do know something about
you can do it your self if you want to try use a diamond cutting wheel to cut out the sink polishing is a bit harder and would seek a  pro to deal with it
As Jason said where are you i know of 2 places in the Birmingham area 1 of them a mate ownes so can point you there way