Gigaclear use, as you said, a direct FTTP (fibre to the premises, aka fibre to the home) connection using only fibre optic cable. They don't require a copper line (BT type twisted pair or VM coax). If you wish to change supplier, a BT style line can be installed if you wished, entirely separate from your Gigaclear installation.
To answer your actual question about Virgin Media, they would indeed need to run a new coax 'line' into your property. Your Gigaclear FTTP line is pure fibre optic cable and hooked up to Gigaclear's network only. It's not reusable for other non-Gigaclear (or Gigaclear wholesale) providers, especially not one reliant on coax like VM. They don't need to dig up the road though, they'll just run the coax through your garden to tap into the nearest CATV manhole cover and hook you up that way. They often used to trench under the lawn or whatever, but in the last decade I've moved house several times and only ever seen cable tacked along fencing or whatever. Quicker and cleaner overall.
The main cable is already laid in the streets, so all they do is effectively run a new 'extension' from the main node into your house. It's a bit like running a new extension lead from an existing socket.
The question remains, however, why you would want to leave Gigaclear. VM aren't exactly perfect, and suffer a lot of issues. If I had the option of FTTP with a symmetrical connection of 1Gbps (or even just their 100/100 or 200/200) I'd be staying put 'til kingdom come!
