A couple of other regular posters on here (who may chip in if they come across this thread) have personal experience of, shall we say, well-intentioned but ultimately DIY, fixes going wrong and causing faults on the local network.
I suspect as far as legals go, section H, 2, 4 of the T&Cs would be as good a place to start as any...
"2. You are responsible for making sure that our equipment is safe and used properly at all times. To do this, you agree to do the following: 4. not tamper with disassemble, misuse, neglect or damage the equipment;"
Adding your own equipment or cabling to that which VM have supplied/installed, could very easily be classed as tampering if it caused issues for other customers.
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