Just to join in, I designed our layout and I think I did a much better design than the professional/sales people. For example, I was told I had to put the sink bang in the middle of the end wall - so drainer offset to one side or the other. This looks shit, Instead I put the sink/drainer combo bang in the middle, dishwasher under the draining board, cupboard under sink next to it and then spacers matching kitchen (5cm ea) to centralise.
I faffed for ages with square paper and little cut outs of different size and shape cupboards as I found the online planner sloooooow and crashy (this was a few years ago).
Key tip - somewhere out there is pretty much any size cupboard! I came up with (and costed) a design for the Howdens range, one for Wickes, one for Ikea, and in the end sourced from some company I had never heard of via my brother in law who is in the trade. Each company had different permutations of sizes. Some co' simply didn't do sizes I needed to make the design look nice so they got ignored.
I identified bits I really want - e.g. a 1m wide drawer stack for plates and my copious cutlery/utensil selection; 1m wide 'landscape' wall cupboards one above the other, and that helped the decision.
I suggest getting a quote from a local independent - thats what I did in the end although I did basically say I want this design, I know I can do it for £x from this supplier, plus fitting what can you do? Good ones should be able to work to a price for you. I got all my own appliances for fitting (online).
Finally! Flat pack takes someone (the fitter) time to put together, it can be cheaper to fit fixed carcass kitchens as they are that much faster to get in, and as it happens are then apparently sturdier once in.
It is technically viable to get carcass from one place, doors from another, worktops another (and then one guy to fit the lot).