I wrote to them and requested them to tell me what was low carb the reply is it is listed on the app.
They had a disclaimer .
"We cannot guarantee that our food or drinks are allergen free.Due to the use of shared equipment in the display, preparation and service of our products there is a risk of cross contamination." keep scrolling down
https://www.costa.co.uk/docs/store-allergen-guide.pdf but the only thing is the melon pot at 6 carbs the rest are very high but most are bread, rolls or cakes so have all the dreaded things to eat as a diabetic anyway.