Unfortunately they like the damp (as we typically have in the UK at this time of year) and will find warmth. So there will be lots about and may seek you out. You either need to repel them from your face, or attract them to something else.
In the garden I use beer traps to draw them away from my plants. They're dishes with a lid like an umbrella, that you put on or slightly in the ground, and add beer. The slugs then drown in the beer. Supposedly it's the yeast that attracts them rather than the alcohol, so carrying a dry mix of yeast and sugar, and adding water at your bivvy site would be worth a try, with a non-food container to hold it. Place it near your head, on the ground.
If you'd consider cosmetic and related products, some may attract them, but others may repel them. In particular various plants, many of them herbs are reputed to repel slugs. Rubbing your face with garlic may not be an option (though I wonder about eating very garlicky dinner). Lavender is one that stands out from this list (at the bottom of the page) as it's used both as a scent, and in natural insect repellents. If you've got any of the latter (I used to have one called "bugs away" that was citronella, lavender, etc.) it might be worth trying some of that. Note that some essential oils aren't great for waterproof fabrics, so don't go splashing it around too much.
I'm glad I bought a sleeping bag with built-in mosquito net. The slugs would be too close for comfort but not as bad as on bare skin.