For some great design ideas, I always look at the Channel 4 website. The design shows - like Grand Designs - have launched a lot of web content that can help you to navigate your way through your design.
I like this comment from the site's 'Design & Style' section:

"A cheap kitchen can be given a lift with a more expensive worktop. It's a mistake to go for the cheapest on offer - always stretch your budget to get the best you can afford"

It's understood that granite worktops can be expensive. It is less understood that they can be affordable - that is to say that they can be adapted to suit many more budgets.
There are high-end projects where the worktops cost over £10000, but for most homes, spending £1500 - £3000 will be all that is needed to give the kitchen that extra (Cliche warning!...) Wow Factor.

So, what do you look for in a supplier?
Remembering that you are spending a fair chunk of money - about the same as a holiday or a used car - you should allow enough time to be really happy with your choice of stone and supplier. Granite worktops will also last longer than holidays or cars, so careful choice is doubly important.
Your supplier should be willing to spend time with you as well. Anyone who wants to close the deal after one call, and tries to hustle you into a decision is probably not the right company for such an important purchase.

A company that has a showroom is usually a good indication - but do try to visit them. Even if it is a long trip, meeting the staff and talking to them in person can pay dividends. It also lets you see what kind of outfit they are. Some 'showrooms' are little more than a ratty old corner of a factory unit. Better to find this out after a day out in the car, than when you are trying to have a problem rectified, and discover that the company are a hole-in-the-wall outfit.

How to choose your stone
Granite is a natural material. It will have colour variation, pits, chips, scratches, blemishes, marks, and many other visible and invisible features.
It is up to you to discuss with the supplier what would be acceptable - there are industry standards, but you need to be aware of how your supplier adheres to these.

Plain black is still the most popular choice. It looks sleek and stunning in the right kitchen, but does show marks. In a busy home with enthusiastic children it might cause a lot of work, and another colour (even other black granites) may be a better option.

Ask your supplier for samples. If you can visit them, pick up your samples, and check how they compare with display pieces and other slabs of the same granite type.
If you are not able to visit your supplier, do remember that granite is heavy, and they have many sample requests - it is not unreasonable for them to ask for a small fee to cover their postage costs. Around £5 to post 1-2 small samples seems to be the norm.

If you put the time in, and find a good supplier who does likewise, your purchase will be an exciting, happy experience, and your kitchen will look fantastic for years to come.