The grey on the western wall looks dark and purplish on my monitor. It may come over as too imposing and monolithic on the southern face. If you're sticking with the grey palette then a warmer tone would suit the sandstone better. Does Dulux have a colour palette recommendation that includes your Berkshire White plus a grey? Check online or in store pamphlets. But, for such a big, expensive, and impactful paint job I would actually be tempted to get a colour consultant. You're not just repainting a loungeroom wall if it doesn't work!

in that case I would stick with the monument. However I must admit that I do love that colour and we are having it on our new build. I like it because it has the richness of a darker contrast without the problem of black fading and looking faded whereas monument can experience a degree of fading over time and it would not be as obvious.

post some photos

Just me -- that's a No ! As it is shown there , its too much , too tacky . On an island , if it was maybe the 'front' it could be okay , or something like a built-in cutting board might look cool ( as it is presumably a manufactured stone , I have no idea how it would be practical-wise ? ) . That whole kitchen doesn't inspire me much -- its too mixed up IMO . Mostly black and white , but with stainless , some timber which is the wrong tone , again IMO -- not consistent .