Fibre is installed in numerous ways. The easiest means tying some fibre onto the existing copper cable, and pulling it through like that. The most difficult means starting from scratch. You'd be surprised at how non-intrusive they can make it.
Odds are that the cable has been there for years, they've just connected it to an exchange. What BT and Virgin tend to do is wait up opportunities, so rather than dig up a road for a cable installation, they wait for the road to be resurfaced and hit two birds with one stone. Of course that cable might not actually be connected to anything, but once the next stretch of resurfacing goes down, they'll connect more or even hook it up to an exchange.
It's pretty complicated, but all you need to worry about is having that socket in your house. They can't simply inconvenience people every time they want to lay a bit of cable, that requires months of planning permissions.