Assuming that you're going to refix the existing skirting:
I use an old flat-bladed screwdriver for this [1] [2]. Drive the
screwdriver into the centre of the plug a short distance with the flat of
the blade going with the grain and then gently lever the screwdriver back,
and this should bring out a section of the plug making it easier to remove
the rest by gently nibbling away at it. [3] [4]
[1] Use a blade that fits inside the diameter of the plug, leaving a
clearance gap between the edge of the plug and skirting board.
[2] If the screwdriver trick doesn't work (fairly often) then use an
equivalent sized sharp wood chisel.
[3] Don't lever the screwdriver/chisel beyond the edge of the plug as
doing so will damage the wood beyond the plug.
[4] If things ate a little difficult, drill a hole through the centre of
the plug (about half the diameter and taking care not to damage the head of
the screw) and then lever/nibble the remainder out. I wouldn't advise
trying to drill the whole plug out as it will be difficult to control the
drill at floor level.
If you're not going to refix the skirting, then using a suitably sized sharp
wood chisel, simply cut a square hole around the plug until the head of the
screw is revealed. NB: trying to lever a screwed-on skirting can sometimes
do a fair bit of damage to the wall surface.
Cash