Choosing a granite company depends a lot on the granite required, what the granite is to be used for plus the location, timescales involved, and what other work is being carried out.
If you are purchasing granite kitchen worktops to go with new kitchen units you are getting through a large company it is possible that the kitchen supplier has their own granite worktop provider and they probably will not be very happy if you say you are getting the granite elsewhere, because they will want to make their mark upon the granite in addition to the kitchen units.
They will possibly make it difficult for you to buy the granite yourself by refusing to return after fitting the units to reconnect services and connect the plumbing for the sink. However, if you are making a large enough saving on the granite it may well be worthwhile to get a corgi registered installer to come and do the final connections rather than using the kitchen fitters again.
If you are only replacing worktops and plan to keep your existing kitchen units it is worth using a granite company with a very quick turnaround time to minimize the time you will be without the full use of the kitchen.
Traditional granite installers need 10-12 days to cut & polish the granite, however, some cheaper companies send the granite to Europe to get the worktops made and then brought back as using foreign labor is cheaper but this means a turn around time of up to 3 weeks. If you find the right company it is possible to get the granite cut, polished, and installed in around 2-3 working days.
If you are having building work done where there is will be a carpenter on-site, it might be that you can further reduce your costs by getting the carpenter to make the templates for granite worktops for kitchens/bathrooms, vanity units, hearths & plinths, and bars.
Also ask your builder if they have any experience of installing granite, as he does and has the right equipment it could be a cheaper option to just use the granite company on a supply-only basis. For this, the builder/carpenter would need to create the templates with accurate measurements and also collect the granite.
However, one thing to bear in mind is that the granite is very heavy and also relatively fragile whilst in transit, so if the builder breaks a worktop while loading it or whilst installing it, it will be down to you rather than the granite company for a replacement at additional cost.