If the wifi signal is poor because of the original placement of the hub and the constraint of a short cable, that's VM's fault, not yours. I don't think that shoving the hub in a corner on a short cable and constraining the wireless signal counts as the "reasonable skill and care" required of service providers by the Consumer Rights Act 2015.
The installers should know that there's often a case for moving a hub a couple of metres to optimise wifi or fit in with room changes, and do what was done with mine, which has about three metres of coax. You have to coil it and use cable ties if you don't need the full length, but means there's always the option of moving it as furniture and room layout change.
Maybe the forum staff can intervene and get a cable sent out to you, if not then search read and follow the VM Consumer Complaints Code of Practice, asking for VM to supply a longer cable since the problem is of their making. Write, or use the web form, don't phone the complaint in - you probably know what a disaster trying to talk to VM on the phone is.
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