Yeah, standard fixtures are something like:
4 x county league matches: Oct, Nov, Jan, Feb
County: early Jan
Southern/Midland/Northern: late Jan
National: mid Feb
(Then the very best county runners get to compete in the inter-counties champs in March, but that's serious bizniz!)
The race distance depends. Normally 8k (local cross country league) to 15k (Southern cross country championships) and potentially anything in between.
As far as I am aware you need to be a club runner or have connections with a club to be able to run second claim.
Yes, yes you do.