I set up an account when rebuilding my cottage, never used them as DIY kitchens were considerably cheaper. Downside is Diy are online only and you have to design and measure your own kitchen, or you did six years ago.
Not sure what the builders red flag is as kichens are pretty standard in sizing, you just choose what suits and fits. Maybe he is worried you order a kitchen that will not fit. Generally, builders will add a 10% to 20% charge on top of the cost price with a supplier they use regularly as they get a discount and basically keep it.