In south shields, Sea Change cafe does an amazing toastie with Halloumi, Onion Bhaji, Rocket and Mango Chutney Mayo. It’s amazing and even better, the cafe is a charity which provides sustainable employment to people with Autism and Learning Disabilities.
Not a specific sandwich but I can whole heartedly recommend grated carrot as an add on to pretty much any sarnie, especially if you need to stretch what you have to cater for more hungry mouths. It’s bloody lovely, plus the moisture gets the whole thing lovely and juicy