50 Meters distance on our Coaxial cable should be fine.
It may be tricky to get the coax cable to the garden room though as it would require either to be ran from the Brown Virgin box on the side of the house or just to have a new line installed from the cabinet to the garden room. Then it would need to be buried and place in conduit.
We would also advise that you do not run any coaxial cable yourself as we use a specific type of coax with the correct connectors etc.
Hope this helps a bit!
Here to help! I'm a technician helping out whilst working from home. Find out more
So my plan would be to pre-install buried conduit with a draw wire from the brown box on the front of the house down to the garden room. Then a Virgin Media installer could use this to simply pull through the specific type of coax and terminate with the correct connectors etc.