One of the classic British tea time treats is a Ginger Biscuit. In America we call them Ginger Snaps- they are very hard, and therefore perfect for dunking but I cannot picture Miss Marple doing so.
Perhaps another English treat, Maids of Honour would be more appropriate- a delicate biscuit with jam, cream, and sherry baked in.
I personally love Shortbread. Anyone who loves butter would love these melt in your mouth buttery wedges. Once, as a girl I was making recipes for cookies that I had not made before and that did not have an accompanying picture. When I followed the directions, I was left with what looked like an unbaked pastry crust and wondered what I had made. I cut the “Pastry” into wedges per directions and baked it – I still remember how good it tasted. It was not fancy like sugar cookies, but it was so tender and buttery. What a memory!