Re: But surely...
The wrinkle is that your TV license is tied to an address, but that address might contain several people who all want to watch BBC/iPlayer, so they either allow multiple logins from a single account, or multiple logins can share one license.
Location based monitoring isn't much use either, because your TV license allows you to watch/listen to BBC stuff while you're outside your house.
Any licensing scheme would have to cope with the totally legit scenario of (for example) one person in the living room, watching TV on an actual TV, via radiowaves (neanderthal), another watching live tv via a desktop computer, a third watching iPlayer on a tablet, and someone else watching via a phone whilst several miles away across town.