We had our DIY kitchen installed last year.
We had plans drawn up by several other kitchen companies, and were 99% certain that the measurements were accurate, but having had an extension built, and knowing that millimeters matter, we got an independent kitchen designer to come and double check. He also suggested a couple of changes which improved the design.
There is a very good Facebook page - DIY Kitchens - Design and Planning Forum and Finished Kitchen Reviews. There are loads of people on there who can offer information and advice around planning your kitchen.
We had decided on a kitchen style and colour, but after a visit to their showroom, we came away having completely changed our minds. I'd advise a visit, as you can get a good "feel" for the kitchen. I think they have opened a showroom in Oxford now - we went to the Yorkshire one.
We sourced our quartz worktop outside of DIY Kitchens, and it worked out cheaper than buying from DIY.
Our other quotes ranged from £33,000, including worktop but no appliances, down to £26,000, again with worktop but no appliances (independent manufacturer, Wickes, Howdens x 2).
DIY came in at £12,500, including 8 AEG appliances. Our quartz worktop was £3,500 and the fitter was £2,500.