Another calculation I have seen (about 5-6 years ago I think) used weightings to allow for population density. This means that the 'centre' would move slightly from year to year, as people move around the country. Because of the general southward drift of the population this point itself has moved in the last 50 years (about 10 miles or so I think).
Unfortunately I can't remember exactly where the point actually is, but I think it's in south Derbyshire, near the borders of Leicestershire and Staffordshire.