madscientist DH and I realised a few years back that we were each spending between £5-£8 per work day in Pret/Eat etc on lunch and that this worked out (at a conservative estimate) at £60 per week going on lunch! We almost overnight started bringing pack lunches and would eat them in the common room of the centre we both belonged to in town, we got a lot of stick from friends but we noticed the difference in our bank balances almost immediately so we didn't find it too hard to break the habit.
One thing I still do (I now get lunch free 5 days a week in work) for DH is to make soup and freeze in individual portions, he can take that to work with bread/fruit and microwave it, because it is frozen it keeps well all day and keeps the rest of his lunch cold if he has a drink etc.
I also make extra when we do paella/risotto/spag bol and he takes a portion to work the next day.
On days when there's no soup or leftovers he will take bread, tomatoes and a little pot of egg mayo he makes up the night before.
In September when he starts his new job he will get free lunch every day in work, which will be amazing!