I've taken out the existing skirting boards and it's left this big gap between the floor and existing plaster/wall. Fixing standard 12.5mm plasterboard to the existing batons would stick out proud of the existing part of the wall. Should I instead fill in the area using bonding/Thistle Hardwall? Then I can plaster over it to meet the existing wall, and pop on regular skirting.
Hardwall*, then skim.
Leave space at the bottom, don't try to make a perfect match to the floor.
You'll have to decide whether you're going to nail your new skirting to the [possibly shortened] original wooden spacers, or remove them & 'No More Nails' the new skirting to the wall. Actual nails are more readily removable without pulling the skim off if you need to get behind them again at any point in future, but glue is quicker & easier to put up.
*Hardwall is just a no-brainer on old victorian brick. Browning will get sucked dry much too quickly.