I have only just realised a subtle titling difference.
The Rider of the White Horse (UK edition) by Rosemary Sutcliff
From a 5/5 Amazon reader’s review:
My sister and I agreed on few things when we were young, but this book was one of them. We both loved it and still do. I have a copy purchased long ago. This book about Thomas Fairfax – an English Civil War General on the side of Parliament – told from the viewpoint of his gallant wife – is magnificent. I treasure it. It moves me and I believe in the authors portrayal of Anne and Thomas Fairfax. They live and breathe within this book – as indeed do all the characters. Neither Anne nor Thomas thought that the King should be executed and Thomas refused to agree to it although the book does not reach that point in history – but their characters are so well portrayed that you can well believe it.
And here some more covers of UK versions: