Whenever anything comes up on forums I am sooooo glad I live in the UK not the US. Here the tipping is for good to excellent service, not expected or even considered compulsory as I find in the US and Canada (and even more so in Russia!).
If I am reading the OP correctly she is going into a coffee shops and buying a drink and ready made snack. What was there to tip for. Do you tip if you go into a bakers and buy a cake, or a book shop and buy a book? The ready made snack is being sold with the mark-up in place and they shouldn't need a tip on top to make a living. Now if you are sitting in there, and they are quick, pleasant and helpful, then yes, a tip may be in order for the person who has served you but surely not for a quick over-the-counter purchase.
Remember this when you are in the UK too. We do not tip as automatically or generously here. My Candian cousin was surprised when my Canadian sister and I stopped her leaving an inappropriately big tip after a pub lunch where the service had been slow and indifferent. Fast food shops, pub food service and coffee shops would normally be no tip or just a little change, 10% is plenty generous enough for good service in a restaurant; any more needs exceptional service to justify it.