I have used a cheap mitreing device which I got from B&Q, you just slide the
skirting board in, two thumbscrews hold it in place and you just saw down
the 45 deg. angle on the edge for an external mitre. Internal corners are
best profiled the trick to that is to first cut an internal mitre then
follow the profile with a coping saw.
If you are not confident sawing mitres with mitre boxes and the like, buy a
mitre saw, Screwfix are selling 2 that will cut deep mitres up to 178mm (7")
for £20 and £27. These saws consist of a hacksaw like saw mounted on guides
that slide down guide bars the whole arrangement can be swung round to cut
almost any angle including mitres.
After that the next best thing is a sliding compound mitre saw. Got one this
summer from B&Q for £85 for a 250mm diam blade with 295mm lenght of cut it
has been ripping through all sorts since don't know how I managed without