I've always been vegetarian too and unfortunately learned too late about the importance of protein. Make sure you have a decent amount of protein to make sure you feel full. Low carb has also been great for me but that's more individual.
Here's some ideas:
Individual mature cheddar portions, oatcakes and e.g. celery sticks, carrot sticks etc. Maybe a few loose tomatoes too if they're to hand? Or a handful of almonds.
A couple of cold boiled eggs (not egg sandwich, just the eggs). Plus a portion of salad leaves in a lunchbox.
Tub of full fat cottage cheese just eaten out of the tub so no prep required, plus a few pieces of fruit e.g. bananas, apples etc.
I'm not a massive Quorn fan but it can be a convenient veggie protein. So those Quorn roast pieces that come ready made, tossed in with salad leaves, tomato, cucumber and a dressing. Be liberal with the Quorn so you feel full rather than feeling full from bread, so a large portion.
A portion of frittata (Spanish omelette) served cool. Morrison's do a nice one on their deli counter if you don't have time to cook. Plus some veggie batons (pepper/carrot etc.)
I'd always have nuts and seeds with any of these lunch options, e.g. pumpkin seeds, almonds, walnuts, pistachios. Lidl sell some of the best nuts IMO. You can portion them out into a snack box to stop over eating them because they are very moreish.
Basically veggie proteins and low carbs!