I almost always buy lunch 'on the go' as I rarely have the time or energy to make a packed lunch in the mornings. I know, I'm terrible.
Luckily, the offerings of grab and go lunch options for vegans has improved vastly in recent years in this country. My most often shop-bought lunch is the Tesco meal deal, as it's on the way to work and I feel represents good value for money. As far as I'm aware, only the falafel and houmous wrap is suitable for vegans as the main part of the deal, then you can choose from a mountain of different crisps, fruit pots and nuts (dependent on where you go....there's more choice in the main stores than the express stores, naturally) and a bottled drink.
I tend to get this trio of tastiness at least once a week, usually more, and grab a couple of pieces of fruit for snacks. I wish more places would take the unnecessary mayo/yogurt out of their falafel wraps (Marks and Spencer's I'm looking at you!)
That said about M&S, they do have some pretty good options for vegans. The main on their meal deal suitable for vegans is the large vegetable samosa which is another thing I get fairly often (comes with a drink and a side). I think there's a soup that's vegan and I can't remember if the Moroccan spiced butternut couscous is vegan or not.....will check next time I am there and update.
They have various 2 for £3 salad pots which are suitable for vegans too, including a slow roasted pepper & orzo pasta, borlotti/flageolet & black bean salad, quinoa/edamame salad with soy & ginger dressing and a super wholefood salad with a mint vinaigrette. Plenty of options!
I haven't bought a Boots meal deal in a while, but I did enjoy their houmous, carrot and coriander sandwich on tomato bread (the Kimchi salad is great but I'm a sandwich kinda gal). They have a fantastic selection of drinks too, including savse cold pressed juices. Their sides range from crisps, nuts etc. to fruit/vegetable snack pots and things like muki beans with houmous.
Of course there are various coffee chains that have some options suitable for vegans (Starbucks, pret a manger) but they're kind of over-priced for what you get plus I don't have time in the mornings to queue to pay for a salad.....self-scan tills all the way!