All the options have pros and cons. Central (UK central not England central
) means that all points in the UK are accessible, although it does mean that the extremities are not for the faint hearted. Starting nearer one end (or side) of the country may well put some riders off entering (although it doesn't seem to bother our continental Europe diehards), and it will almost certainly discount some areas of the country from most riders, but does mean that some of the, normally lower scoring areas of the country may well attract higher scores.
Overall, I'd like it to start nearer the middle of the UK in travel time terms, not geographical (i.e. a line from Carlisle to Newcastle), but no matter where it starts, I'll be there next year.