Well if it were me I'd stick to what she likes but introduce new bits alongside it each week and put the new things on rotation and don't worry about it too much. So easy to get caught up in teh my child eats badly thing when actually she propbably eats very well, for a kid. They like the same old things. So do most adults tbh. I have pretty much the same 3 things every day of the year for my luncha nd breakfast. Nothing wrong with that really. As long as we all eat enough fresh fruit & veggies.
Anyway - some ideas -
cut sarnies into shapes - make it more fun. Theme them as well if you like, So a cookie cutter in a heart for Valentines week, a Xmas tree cookie cutter at Xmas, star at guyfawkes etc. Try and make sure it's always stoneground wholemeal but if she hates it use 1 slice white and 1 slice wholemeal and make sarnie that way
Sarnie content - a tiny bit of grated cheese (i know she likes it cooked but put it in there now and then and she might just go for it. Add a bit of marmite to it or sometimes try a bit of grated carrot in there with the cheese, a bit of thin ham
cold pizza
I give my DD spag bol for tea and then keep a bit back for her lunch box the next day, she loves it
mini scotch eggs
hovis crackers, maybe try some of that smoked cheese that comes in those little sausage shapes, grapes, breadsticks, , carrot batons, bits of dried fruit, all in the same little pot in her lunch box
a healthy yoghurt - my DD loves the activia ones with prunes in the bottom (only she seems to think the prunes are chocolate, he heeeeeee!!)
I get these dried apricots covered in yoghurt from our health food shop, they go down very well and the girls think they are getting sweets!
Fruit and cereal bars. You can make your own easily. Nigella has a great receipe for breakfast bars, just omit the peanuts for school (ask if you want recipe)
some boiled new potatoes with bits of sausage cut up and mixed with mayo or sour cream or natural yoghurt