You could grab a pre-made plain salad from whichever supermarket and then get some ham/chicken/something to go on it?
Or even grab a Subway salad. These are free: beef, chicken breast, beef, ham, chicken tikka, chicken teriyaki and turkey breast
The sauces are:
Sweet onion – 1½ Syns per serving
Honey mustard – 1½ Syns
BBQ sauce – 1½ Syns
Ranch dressing – 2 Syns
Light mayonnaise – 3 Syns
Chipotle Southwest – 4½ Syns
Just ask them to pile your bowl high with the lettuce, cucumber, etc. and you're sorted