Hello slimey-c and welcome to V-Flyer

You can upgrade any of the VH booked VS fares with cash either before you travel (via VH) or at the airport (via VS) if there are seats available, of course.
There's not much difference in price (depending on the route) between doing the upgrade before or at the airport, but you have a better change of getting the seats you want if you upgrade via VH well in advance of travel (especially on LGW routes).
It's worth pointing out that even if VH dont have any more seat allocation in the class you want to upgrade to (if trying to upgrade with cash before you travel) you may still be able to upgrade with cash at the airport as you are then dealing with VS at that point and not VH, so they are likey to want to sell any remaining empty seats on the A/C if there are any.
VS are unlikey to turn anyway any cash upgrades at the airport, regardless of who you originally booked with. However, if there are only fully flexable fares left on your flilght (ie the flight is nearly full), you might find the cash upgrades are very expensive as you are paying to upgrade to a fully flexable fare.
Upgradeing with miles is a different matter and cannot be done at any time.
Regards