yes steve mate you got it pretty much right. the 5 man for less experienced teams and the 7 man for the more established teams. although as i said before it is open to everyone that just a general rule of thumb with regards to the teams.
The UKPL i understand will have teams from all the series in it. i.e. series 2004, CPL, M25, and the SWPL. it's something like the top 3 teams in each division or summat tho i can;t be sure as i don't have all the facts on that matter mate.
hope that helps.....