I'd fit the panel to the wall and cut the skirting. That sounds easier because the panel is presumably flat, whereas the skirting is a complicated shape that you'd have to cut into the panel. Also the skirting is cheaper to replace if you bugger it up.
Installing bath panel and today and the skirting which will meet perpendicular to it.
Shall I run the skirting past the point where they meet and cut the panel to fit or fit the panel to the wall then cut the skirting to where it meets?