We had this problem when we moved into our current house, Victorian with air bricks, slugs were coming in through the air bricks up under the floor boards and into the living room, disgusting.
I solved it by simply putting stainless steel wire wool in the air brick holes, put a bit of vaseline on the edges of the hole and spread the wire wool out nice and thin so the air can still flow through but the slugs cant get in, you can get it on the web for a few quid, simples.
Homebase also sell mesh grills for a few quid that you can put over your air bricks, just put some sealant round the edge so it lets the air flow but keeps the slugs out, did that to the air bricks at the back of the house, still notice draughts coming through the floorboards when it cold so the house is defo still breathing properly.