You can try calling in, and booking a "non-fault call out", which will be £25 (and usually well worth it), or if you hold on here the forum staff may be able to book one for you, but can sometimes take a few days for them to get round to replying.
but presumably you also need some further VM equipment as well for TV, unless you are planning to relocate existing kit to the extension. The wi-fi is provided by the VM hub and you only have one of those per customer so wi-fi coverage will depend on where your existing hub ends up being based relative to the extension and the other rooms in your home.
You can phone in to book the cabling work. Forum staff on here can also arrange that but they don't often visit this 'Tech Chatter' forum.