How to Avoid Being Tipped

Do not acknowledge you two new customers, sitting at the table closest to the till point where you are standing.
Walk away into the kitchen empty handed and return ditto.  
Do not make eye contact..
Make sure that you have virtually covered the small sign saying "Please Order at the Till" with a huge piece of board.
Watch the customers sit waiting with menus in front of them.
When the female notices your sign and helpfully tells you it is obscured, do not acknowledge her at all.
Deliver the tea and cakes in silence.
When they get up to pay, on no account speak more than to utter the total bill.
If you follow the above instructions with care you will avoid the embarrassment of being tipped which would be demeaning.  Anyway, you didn't serve them at all so you have the moral high ground
(Extra points for working in a very attractive coffee shop in Matlock where the single unisex loo is in the kitchens and one of the other customers was a young dog which did a fair amount of howling.  Good job it's not a tourist area tho')