There are a lot of variations between houses and kitchens. For example, gas and water plumbing can be trivial (if open space underneath or behind the kitchen) or very hard (some houses in my neighborhood have the original electric kitchen on a concrete slab so getting gas into the kitchen - even though they have gas heat & hot water - is very expensive). You may have a lot of electrical work to be done that was grandfathered in but now that you are renovating would have to be replaced (this is very common in the USA), etc.
Unless you are in a "no way I could possibly hire a professional to do the work" mode (in which case I think it would be dishonest to bring in a professional to do a free estimate), I would consider finding a construction company that does this type of work and see if they can either:
- Give you a free rough estimate (i.e., not draw up plans, but take a look and give you a rough idea of 'x' for the plumbing, 'y' - 'z' for cabinets (will vary depending on style and quality), etc.
- Do a formal assessment, including drawing up some plans, for an agreed price for a few hours of work.
One other possibility (I did this myself) is to go to a big-box store (Home Depot or Lowes in my area) and have the kitchen department draw up some plans. They will generally do that with no obligation, with the understanding that you will likely buy at least some of the project materials from that store. If you go this route, measure everything first: Dimensions of existing cabinets, location (old & new) of sinks, stove, etc., location of walls, etc.
As far as "flat-pack". That depends on what is available. When I renovated my kitchen (~ 19 years ago), the local Home Depot stocked a full flat-pack brand (Mills Pride) and I was able to get almost everything ready-to-assemble at a very good price. I only had to build two cabinets from scratch (and I was able to get materials to match the flat-pack cabinets). But now they don't stock it any more, so the same project would be a lot harder. So the specifics will really vary a lot depending on what is locally available in your area, your budget and other factors.