Hi Mike
We get an average of 15 pilots at our dometic competitions. Our pliots briefing is usually 8:45 for first flight at 9am. The maximum number of entries we can cope with is 21 pilots flying 3 rounds, but some would fly Clubman and Intermediate which are shorter schdules than the 'P'. We would never have more than one flight line, (which we experienced when Matt and Richard flew at your Nats this year). You can see numbers for yourself here:
http://www.gbrcaa.org/competitionresults.htm
Most pilots live within 120 miles of most competition venues although the odd pilot could travel 200 miles. I would guess the average might be 90 miles (180 miles return).
I hope this is helpful to you.
Kind regards
Ashley