They are doing my kitchen as we speak The basic kitchen refit is a week, kitchen and new appliances delivered on the Monday and finished on Friday. If worktops need to be templated once kitchen is in (granite or quartz) they put in a temp worktop until the new one is ready to be fitted.
As we were changing layout and removing wall cupboards and all the tiles and having some pipes boxed in etc., we had the old kitchen taken out on the previous Weds and then boxing in and replastering done. This needed a few days to dry out before the new kitchen could go in. JL sorted all this out, but we had to pay the plasterers separately.
We started off looking round the kitchen dept and picking up their brochure, having a chat with one of the kitchen planners. Once you have a good idea about what you want you make a planning appointment which costs £50, redeemable against the total cost. They come out and measure up and do a quote. Then you can change/add things until you're happy with everything.
They use independent kitchen fitters - in our neck of the woods, the kitchen fitters IKEA used for their fits also do John Lewis. So you won't be dealing with John Lewis, you'll be dealing with the third company as well. Whilst the actual fitters and the quality of work were excellent, the back office management left a lot to be desired. I'm not sure that just because they were working on behalf of JL you would get better service.
Ulitmately we were very happy with the fitters, but bear in mind JL use contractors.