The protagonist is a girl who finds a key. The key unlocks some sort of door that allows her to time travel. She uses this door to meet important historical figures, one of which being Henry the 8th.
I know it isn’t much to go by, but I have a possible lead on a book titled “Henry the Eighth’s Hat”.
Some further information – I’m trying to locate this book as a gift for my mother. She read it in her childhood (she’s in her 50’s now).