Having done my own oil-services for years, I'd always advocate the sump plug out / drain method, if only for peace of mind! But also, as Rob rightly points out that you just don't know exactly how much of that dirty oil is left behind unless you meticulously measure out what you've extracted. Can imagine there's lots of 'pooling' going on all over the engine internals though.
Not a fantastic hassle if ur handy with a jack and axle stands or can find a friendly garage willing to lend you a pit or lift for half an hour.
I'm lucky enough to be located by a DIY garage here in Holland where you can rent a lift by the half hour at minimal cost. i'm sure they must exist over there in the UK...?