You need to do the island & your other floor cabinets first - they shouldn't be installed on top of a floating floor. Particularly the island as it won't be attached to the subfloor & will therefore not be stable enough. The floating floor can then be installed around the island. Leave the skirting boards off, so the floor can go under the cabs as far as possible, then install skirts after completion of the floor.

The volume builder's offer it cheap (or standard inclusion) because they bargain down the supplier and installer who will be suppling many (i.e.: volume) bench tops of pretty much the same size and specification..... Custom designed/bespoke/renovation dwelling's cost so much more because they're 'one off' rather than the same design being built many times over.

if the space is at least 4m x 4m then something like this should work but as soon as you have more precise measurements we can make adjustments and maybe make space for a washing machine inside as well? the tall shallow pantry cabinets could be custom made or if you have access to IKEA or similar there are flat pac suitable cabinets but they must le less than 40cm deep to allow enough walking space with a 90cm island OR the side cabinets could be deeper base cabinets plus more wall cabinets and the island narrower eg a 75cm wide island and 60cm deep side cabinet with walkway adjusted to suit and/or the parts of the island could be like a table so that stools could fit under completely and be out of the walkways when not needed

I'am not sure why your builder suggests that a 900 oven needs to be built in under bench, 900/ 600 ovens can be mounted in a wall cabinet. After having a 900 oven I personally prefer 2 x 600 I have shown you as best I can a single 900 oven wall mounted plus a freestanding unit. The 900 wall oven can become 2 x 600 stacked ovens In my experience MW's a better sitting in a special Wall mounted cabinet above the bench, it puts it at a safe height for handling hot foods, easy lift out onto benchtop. I still prefer the Fridge set back into the wall beside the WIP, family having BBQ's and just easy to access with ease of loading and unloading it by using the Island. The WIP I have shown without return corners as I dislike them as they create base cabinets that are difficult to access, it also allows for a full height window for extra light and ventilation with the straight run You can have a double sink but I have shown 1 1/2 bowls, which I find easy to work with especially if you have the newer deeper square sinks, you could have a dishwasher but I prefer that next to the sink below the servery. Your WIP is perfect for all small appliances, coffee machines, Airfryers etc. A tall food pantry would be a good option between any wall oven and MW unit, the pantry would be best fitted with drawers. I have made the opening into the WIP squared and full height, if you use standard height and trim any ovens etc would need a filler between it and the wall to avoid hitting any doorway trim .