At least:
Two types of finger sandwich
Another mini savoury option such as mini quiche, quality mini sausage rolls, a mini pastry sliceetc
Crisps should be available
Two scones per person, fruit and plain
Real butter, properly softened
Proper Clotted cream
Top quality strawberry jam
Two further mini sweet options, probably three - something fruit based, something chocolate based and one other. So lemon drizzle, brownie/ chocolate pot, carrot cake/posh Florentine.
For drinks, choice of three or four teas and a couple of coffees, ground from bean. I'd also always have still and sparkling mineral water available, and a good prosecco too.
If I'm catering afternoon tea properly you're not going to want to eat until lunch the next day!