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In terms of the volume of the final beverage, 1kg of instant coffee goes a lot further than 1kg of whole bean coffee does.

I don't measure coffee by weight when making the stuff (I use the eyebolic method with the coffee grinder, and the equally-imprecise heaping spoonful method with instant), but some homework suggests that it takes ~60g of whole bean coffee, or ~9g of instant coffee, to produce 1 liter of beverage.

If a coffee cup holds 300ml (as the normal US-centric one I picked from my cabinet does), then when we use your prices we get:

~£0.702 per cup from premium, locally-roasted, single-estate Columbia coffee beans

~£0.1134 per cup from 'premium' instant coffee.

...which makes instant a whole heckuva lot less expensive to drink. :)