|Nicey replies: Greg,
There are no hard and fast rules about the choice of biscuits for any given occasion. However, it is perfectly possible to choose biscuits well or in poor taste depending on the elegance of the occasion.
I would agree with your first choice and would also add such things as Malted Milk, Fruit shortcake, and possibly some All Butter biscuits, depending the numbers present. At such an informal gathering of friends it may be nice to try out a new biscuit and so almost anything could turn up.
As for a dinner party I would be minded to chocolate Hobnobs plain and milk. Something decadent like a McVities Boaster could so easily form the center piece of a culinary evening. Serving Iced Gems at such an occasion, however, would almost certainly end up with you being ostracised by your own dinner guests.
As for a hired out venue one would always expect to see double layered biscuits and as such the jam and cream sandwich, which would probably head up a plate that includes the custard cream and possibly the bourbon. At such an occasion it would be bad taste to serve up wafers or anything in a branded wrapper such as a Penguin.