Hi
I currently have a mk2 box and going to full fibre will I have to use a cat5e cable to connect to router or can I use existing cables
Anything less than cat 5e cable is not rated for 1Gbps. Your choice.
When you move to full fibre your existing sockets and wiring are redundant.
The fibre will terminate on a new ONT (optical modem) which connects to the hub via an Ethernet cable.