When I first started buying coffee from Starbucks, I sometimes had difficulty getting the right strength too: apparently filling a large mug with a mix of filter coffee and espresso was new to them then. They have since adapted (though my brother ordering and drinking 24 shots still caught them by surprise...) - I am sure asking for a filter coffee diluted with hot water would be fine. ("Grande in a venti mug, topped up with hot water"?)

Hot chocolate with marshmallows ... Costa's saving grace, in my book! Lousy coffee - I don't think I've ever tried their tea - but that hot chocolate is quite good.

Multinationals doing scones with jam and clotted cream ... I'm sure I've had exactly that in Tesco in the past, and they should qualify; Morrisons may not qualify as 'multinational' (though Safeway would) and certainly do scones with clotted cream in the cafe - even when, to the bafflement of their own staff, the main shop didn't stock clotted cream at all. Some of the Starbucks scones can be good too, though sadly no clotted cream on offer there.