Our kitchen is NOWHERE near that size so I can't imagine ours should cost anywhere near that
Our carpenter (who has fitted most major brands) had a little fit about it. He reckoned it was the best quality he'd ever fitted- went on and on about how good the legs were. The whole thing only took 3 days and one of those was making a couple of bespoke cabinets (they come ready made).
We have a strict budget of £6k max
What do you want in that? Looks like you are going for about 8-10 cabinets? Maybe £3k in the type you want from diy kitchens. Say another £800 for plinths, corners etc (I think I'm overestimating on this, to be on the safe side). £300 for an oven and hob? £200 for a dishwasher. £200 for a fridge freezer. Say £1800 for fitting with a skip to remove the rubbish. Worktops will be bugger all (£500) if you go for wood or laminate, but will add a bit if you want granite.
I think you will need to be a little careful to come in under that price, but eminently doable. Appliances, worktops and fitting are where it might go up. Fancy wirework in cabinets or big pan drawers can be expensive.
I'm sure you can haggle B and Q down to that cost all in. The plus side is that it's all in one and you don't need to source a fitter. The down side is B and Q stuff feels somewhat cheap and you are going with the appliances they have.
As you say, it would be a little more work to get the fancier kitchen at the same price. I'm a bit of a control freak, because ours will be there for years- each to their own.