Installing a box in a summer house is not a typicl install.
I would expect the customer would have to run conduit from main house to the summer house for the technician to pull the coax cable through and install ethernet if the summerhouse has its own electrical ring mains
It would have to have a separate ethernet run out to it, Powerline adapters require both adapters to be on the same electrical circuit. An electrical supply to a summer house would be afaik on a spur or dedicated circuit from the consumer unit, and would have to be protected I believe by it's own RCD
Let us try and enlarge on these answers to the question.
Each account at any one address can only have one VM Hub to bring in broadband and then provide wifi. It is usually the case that the Hub and the separate TV entry point are co-located but they don't have to be. For example (and not unusually) my TV is located downstairs and the Hub immediately above it in the room above it to provide broadband and wifi on the first floor primarily.
You can have more than one TV box. For each additional TV box (total of 3 boxes) you have to have a separate connection from the street feed to your house to the location of each box. VM have to install these. But they would not be willing to run excessively long co-ax cables from the entry point to an outbuilding, especially if that means going through the house, unless you want holes drilled everywhere and cables running on the surface of your internal walls. VM techs are not building experts.
If you have one or more VM V6 TV boxes they need to be connected to the Hub. The best connection is via an ethernet cable. But you can connect the two via wifi and that often works well (for multiple boxes), but not always. In those cases you will see, if you read posts on this Forum, that people often use power line adaptors to bring the internet connection from the Hub to the TV box if they are not co-located. If you have the ordinary TiVO (predecessor to the V6 you have to use power line adaptors. There is no wifi option.
To get internet from your Hub to the summer house you can look at two options. The first is to install a length of Cat6e internet cable from the hUb's ethernet ports. You can run quite long cables without any deterioration in broadband strength. You'd normal terminate the connection with an internet switch so you could run more than one internet device from the connection at a time. Say two laptops if you and your partner wanted to be in the summer house at the same time. You can also try a wifi set up. It is unlikely that the wifi signal from the Hub will reach the summer house without some help so you are looking at a mesh system to get the signal there. Look up the terms.
As for VM TV I doubt you'd get VM to run a separate co-ax to the summer house (as I have said) but there is an alternative. Use your laptop for BBC, ITV, C4 and C5 programmes via their respective players. And for the rest look at VMs TV go app on a mobile device.
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For another V6 the tech would have to split the cable again and run a length of coaxial cable to the summer house. One question is what would be the length of the that cable as there would be signal attenuation from both the splitter and cable length. A summer house at the end of a long garden could mean that the signal strength would be too low.