YA mystery features english girl I read around 1960-1965

Read in st. Joseph missouri public library around 1960-1965. No jacket. Fairly thick book. About a young British girl who goes on an adventure by herself to solve a mystery. More than 1 book written in a series, I believe. Common British terms used such as "tin of food" and "rug" for blanket. Not Enid Blyton. I've looked at those books. Thank you in advance.