Sorry to be the bearer of bad news but you do NOT have optical fibre broadband. Your HUB3 box is connected to the internet using a F-Type Coaxial Cable which is a cable with one solid copper wire inside and then later connects to a fiber link outside often hundreds of meters down the road, this is known as a HFC network and is not classied as FIBER other than the abuse of the wording by the internet companies.
I have managed to increase my speed by buying another router, I had a local guy come and he assessed my setup and supplied. fitted a TP Link archer router and configured a load of settings which instantly brought my speed from 65,33 download to 174.49,,,,,, the upload went from 18 to 20.
I live in a rented house and the landlord had the engineer install the router in a place where I can not hard wire, he will not allow cables to be run around the house, my own local engineer fitted in the TP link plus gadgets that fit in your wall sockets which I can haed wire to (which I am)
At least I have no more drop outs or have to wait hours for my youtube videos to be posted,
But it is a bit naughty of Virgin to advertise these fantastic speeds when they can't produce half of what is advertised, plus that you have to buy another router to receive a faster and more stable connection, they should supply a router that works as advertised,,,, most complaints on here are about router problems, surely Virgin can listen to their customers and sort it out.