It's a hard one, I love costa and starbucks paninnis etc and so have stayed away in the main since starting SW!
I tried the costa light the other day, and counted is as a HE - nice as like a latte with extra froth and flet like i was having a proper coffee. If synning it I think the small one is around the 3 syn mark.
In terms of food, what you could do is have a few less syns each day for the week and 'bank' them, and treat yourself to something you fancy? Or just flexi syn in and get back on plan the next day as you look like you have lost a lot of weight already, I don't think the odd treat will really slow your weight loss down.
Or, have you considered moving to somewhere else for your saturday treat? It needn't be that expensive - you could go to a pub for example where they will normally have jackets a plenty or you could have steak and pot, salad etc