In slight difference to Sunnyshores post I would put the primary outlets into this order
Shed/Entry level kitchens -
Ikea, Homebase basic, B&Q, Ridgeons, Jewsons, Tesco, Wren, Symphony and the contract ranges from Howdens/Benchmarx and Wickes
Low Mid-range Kitchens -
Magnet, Benchmarx, Howdens, Wickes, Betta Living, Wren, Mastercraft, Kuchenhaus
Upper Mid-range Kitchens -
Premier Kitchens, Moben, John lewis, Porcelonsa, The Kitchen Company, KBBC plus a number of independent companies
Higher level Kitchens -
Seimatic, Christians, Poggen Pohl, Plain English, In-toto, Smallbone, Roundhouse and a number of independants.
Not an exhaustive list by a long shot, there are hundreds and hundreds of kitchen outlets out there, but these are the prime contenders.
The 5% rule is not a bad one, although there is a little variety dependent on location as the average London property doesn't necessarily need to have the 5% spent to get a kitchen that suits the value.
If I had to pick from each category I would say the Homebase's better kitchens, Benchmarx and Howdens all produce good quality units for the price at the lower to mid end. John Lewis kitchens and service outstrip the others in their group and I love Roundhouse and In-toto for the lower top end products. I would have a Smallbone if money was no object though.
In terms of difference and quality the main points are the cabinet construction and material, the quality and finish on the fascias and decorative items and the type and quality of gadgets/storage solutions and associated toys.
Like a car at first glance they all look of a similar nature and are designed to do the same job. Look under the bonnet and around the interior and you start to see where the differences are.
I would suggest having a look around the showrooms of a few from each level to get some ideas of the style you like and preferred quality.When you have a rough idea call in a few companies to design and cost your kitchen for you.