Also note that the CAT terminal only accepts checked luggage from certain airlines (those within the Star Alliance, IIRC)

Yes, but you can charter a private jet there if you can afford it, e.g. privatefly.com/private-jet-hire/…

Oyster cards are not valid for travel between Oxford, Reading and London.

If there’s a map inside the station this really helps. Often though, there are several exits, some distance apart, and labelled only by the name of a nearby street (which could be a cross-street, parallel street, or not even that nearby)