I always thought that it was regionalised but staggered so that in theory anyone could draw anyone else.
Start with, say, 32 balls of the most northerly clubs, then when you get down to 8, throw in the next 24 and so on.
The order that they come out of the draw always follows the same order in the early stages, starting with Morpeth v Ashington, heading down, and then going east to west along the south coast until you get to Truro City v Bridport.
Guernsey used to be counted as if it was based in Gatwick, so I suppose Jersey will be similar.