@wehaveafloater
True on the sizes; they can always be accounted for if the designer is aware of them. I've quoted against countless designs from other retailers before. Nothing is particularly insurmountable; it typically means (at worst) adjusting the size of a cabinet or filler slightly to accommodate thicker/thinner panels.
Sizes of drawers can be tricky occasionally, maintaining the right "lines", etc.
I'll be honest and say that the last part of that is decidedly unlikely though. I'll give you a bit of an insight into the industry.
Every kitchen retailer has costs and needs to make a profit to survive.
Simply put; we have to factor in ALL of the costs; the manufacturing/purchasing, the transportation, the storage, the running costs of the showrooms, the wages of the staff, etc.
Then we have to add on a buffer for potential remedial actions (replacing a faulty door in 6 months time or something like).
After that, we have to include the cost of any finance options that you've chosen; Interest Free Credit, Buy Now Pay Later, Interest Bearing Credit; we pay the finance companies when you take one of those out, so we have to pass on that cost to you.
Finally, we add on our actual "profit" from the sale, once all of the above is taken into account.
That said; bartering is definitely a thing. The first price that you're given will never be the best price that you can get; but you'll never get below the "minimum" margin.
No manager out there would allow their designers to sell kitchens that are unprofitable. Business is business after all.
Sorry, I rambled a bit there but the crux of my point is that just bringing a kitchen design into a showroom will not guarantee you a discount.
If you want the best deal out there, you'll still have to do the usual dance; spend some time getting multiple like-for-like quotes from different retailers and play them off one another by asking them to price-match each other.