Thank you for your post and welcome back to our community forums. We're here to help.
As @japitts has advised, if you need some assistance with the cabling and reinstalling of the equipment in the property then we can arrange a technician to come out and do this for you, but there would be a cost of £25 to do so as detailed in our Price Guide.
Let us know if you need to help you arrange that, as I understand if you're doing an extension it might be best to wait until that work is complete first.
The cable can be split anywhere, even externally, it just depends on the run to both boxes. If you want a cable to be under floorboards just install a pull rope. If you want the mini upstairs you can drill the hole through the ceiling and floorboards now. Note that the mini will need a connection to the hub so the same hole can be used for the coaxial and Ethernet cables.