Sakura, treacle tarts were very popular in Birmingham when I was growing up and I didn't mind them if I only had a small piece. We had them at School Dinners often and Mom would get out the Tate & Lyle's and make a tart on a Saturday sometimes.
I wouldn't say that butter tarts, as we know them in Canada, are unknown in Britain but they are not a popular item like they are in Canada. When I first came to Canada in 1963 and lived in Peterborough, Ont. pop. 50,000, I was amazed at the variety of biscuits, squares, cakes and tarts available both in the shops and made by people at home, especially at Christmas. They certainly experienced rationing of food stuffs during the WW2 in Canada but nothing on the scale of Britain, so I am assuming that they were more or less always able to make such goodies. My husband, although born in London, England and was raised in Vancouver loves all those goodies especially butter tarts.