Most restaurant marinades have a lot of sugar in them, garlic prawns will be cooked in a lot of butter and the chicken they use is normally the fatty bits (its cheaper). Maybe ask for a piece of grilled fish instead of battered with a salad on the side and no dressing? That tends to be my choice.
Be careful of indulging in wine as well. You'll find a lot of pubs or restaurants will add a jus or sauce that is alcoholic and therefore sugary. Eating out can seem like a pain but you just need to be aware of what to look out for.
Fish, lean chicken or even a burger (depending on the quality of the restaurant) without the bun will do the least amount of damage I've found. Or go for a veggie burger if they don't use potato as a base.
Of course the other option is to class it as a cheat day and just go on as before. Treat the next day as a PV day and your body will sort itself out.
Hope that's helpful!