DOCSIS 3 is a slightly different kettle of fish, in the case of DOCSIS 1/2 VM only run 1.0 and DOCSIS 2 can accomodate 1/1.1 modems anyway. In the case of EuroDOCSIS not really VM's problem and if modems were available 'off the shelf' in the UK they would likely be both anyway as we're in Europe.
Again no real technical reason and more a matter of policy, VM wanting to control the network right to the CPE.
In the US customers tend to be able to do what they want to with their cable line, where it his the house being considered the demarcation point between cableco and customer and anything goes after that with it not being the cable operator's problem if the customer breaks stuff and a customer who makes a hash of their home cabling has a return path filter put on their drop meaning they can't interfere with the network. Of course they do lose their return path but minor issue
