One final thought.
A normal 100Mbps ethernet connection (not a Gigabit connection) only uses one twisted pair of wires. Cat5, 5e cabling normally contains two twisted pairs in case you want to use Gigabit. You can buy some cheap unpowered adapters that plug into an ethernet wall socket at each end and allow two separate 100Mbps connections over a single cable. A simple clever idea.
Using ome twisted pair solely for the BT TV box and the other twisted pair for all other data is I suppose yet another possibility?
I haven't tried that either! 🙂
Try a web search for 'ethernet economiser'. That seems to be an official name for them.
You have to use them in pairs, one at each end, and they are designed to be plugged into ethernet wall sockets as normally found in offices.
Note that if rigging up ethernet cable between wall sockets you would normally use a cable with solid core copper wires. That is because the push-down connectors in the sockets are designed to work with solid core.
An ethernet patch cable with plugs already fixed on each end will often contain stranded core wires to allow greater flexibility. This type of core is not really suitable for punch-down connectors.
Just something else to be aware of.