Thinking about this further you probably don’t even need an extension cable simply take the two aerial leads out of the back of the bt box and join them together.
There must be a cable from the bt box to the tv and that’s the one to connect to the aerial cable. So you simply unplug the aerial cable from the back of the bt box and unplug the cable that goes from the bt box to the tv and join them together.
Ah right I use both hdmi and aerial connections from box to tv, in which case you will need to connect the aerial to the tv with an extension cable attached to the aerial cable that is currently connected to the bt box.
