As a broad rule of thumb, you're looking at £100 to install/activate/supply the additional box, and then the box rental probably similar to whatever you're already paying. Prices can vary based on your account, this is something you are best calling to confirm.
The issue you will have, is that in the current climate, anything more than basic internal cabling works is (rightly) classed as non-essential work and is not being progressed. It sounds like you will need new internal coax runs, and the chances of this being done at the moment are (at best) slim.
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