Actually in the station? Or just outside?
Across the road, there is a Prezzo...and a short walk takes you to Kings X and St Pancras, which have soooo many places, due to Eurostar. Everything from Carluccio's to Patisserie Paul.
In Euston, there is a BK, a pub style place, M&S...few other places (EAT, I think) and most importantly...Krispy Kreme! Oooh, they have a Patisserie Paul too...amazing sandwiches, quiche, French savoury delights...and sweet delights. Lovely macaroons and chouquettes!