I'd go for something 45cm deep which fits a standard dinner plate and a glass. That leaves you 3ft which will obviously be less when someone is on a stool

I think you see bars (serving beers) with stools and no overhang. These are fine for a social setting where one person might be sitting with their back to the bar chatting to others in the room. If you intend to use the bar for food you need to face the island so will need the recommended leg room. Could you swap the 600 deep cabinets for 300mm deep? The cost of new carcasses might not be too bad. You should also talk to your kitchen fitter as he may be able to install them after the quartz.

Not sure if you are renting or you own the property. If renting then you will not want to drill holes in the walls, especially if they are tiled. Here are some possible thoughts for both eventualities... https://www.roseandgrey.co.uk/brass-five-tier-display-shelf https://www.rockettstgeorge.co.uk/curved-brass-shelving-unit.html https://www.amazon.co.uk/HGT-Bathroom-Glass-Shelf-Brass/dp/B00ANGO2FU https://www.victorianplumbing.co.uk/miller-bond-brushed-brass-glass-shelf-8702mp1 If you’d consider black freestanding: https://www.wayfair.co.uk/furniture/pdp/chateau-chic-narrow-134cm-etagere-bookcase-chte3415.html

I think Minnie's right, you have 1m of prep space and that includes any space you want for a drainer for the sink. I think you need the extra space at the back, not maybe for the prep, which you could do in the 1m between hob and sink, but for the kettle, toaster, coffee maker maybe, preparing drinks etc. The space is very tight and I think the storage is quite limited. I put the Fridge Freezer in the big grey cabinet, so you could see the difference between it and the rest of the units as it juts out. This i think can be improved upon too:- I would integrate the Fridge Freezer as the space is small and will look a lot better with a flat profile:-