You are dealing with two problems here. One is the merry-go-round of VM's installation processes and the other is gaining the necessary wayleave/permissions to bring a VM cable from the the street to your flat.
VM's installation procedures are overly-reliant on automated processes so, when one installation fails, another automated cycle is started. From the past topics on here, quite often no human intervention happens and, even when it does get to a person being involved, they are unable to break the cycle of automated tasks taking place. Hence you keep getting failed installations followed by new installation dates which also result in failure.
The second issue might involve several different parties depending on who-owns-what in your building. The freeholder/owner of the building will probably need to grant permission for holes to be drilled and cables attached to the building. If the cable has to cross a garden at ground floor level, then you need to find out who owns the garden (which might be the ground floor leaseholder or freeholder for shared land).
The wayleave T&Cs from VM which appear online do not seem very advantageous to anyone accepting them. If that person does not benefit from the VM service, there would not be any particular reason they would want to sign such a document (even if you could find who the right person is).
Estate agents' and letting agents' default answers to such requests are often likely to be a 'no' as they will have no particular interest in getting involved in what might be a complicated and time-consuming process.
One of the VM forum team may be able to add something more when they get here within a day or two.