Hi Lewis
In all honesty, getting pre-season friendlies is more down to who you know. From my experience, at Contrib/Supply level and above, lots of clubs have a contact in the county FA or referees' association that approach a friend/colleague of an appropriate level to referee the game. The referee then puts a team together for the game, frequently made up of his/her refereeing friends/colleagues.
At the local level, it might be worth sending a few emails out to league referees' secretaries, who are often approached by local Saturday/Sunday clubs for a referee.