Flask of syn-free soup with crisp bread of HeB bread?
Huge tub of salad with tuna, hard boiled eggs, chicken, ham etc. Tubs of cottage cheese generally low-syn.
Pasta salad made the night before - lots of veg. Pasta and sauce in a wide-necked flask might also work.
I think someone on another thread recommended mug shots - some of the flavours are free/low syn I think.
I've never tried it but I've often wondered whether you could keep a jacket potato hot if wrapped in foil/insulated bag? Could take baked beans in a flask?
I take a massive bag of fruit with me to work along with a couple of Muller yogurts. Keep a packet of Alpen bars at work in case of munchies and some sachets of Options hot chocolate in case you need chocolate.