I remembered there was a thread about this years ago, so I googled for my own response - and here it is, copied and pasted, from 2016!
"Oh, memories!! When I was tiny: the Tommy the Tugboat series by Marjory Beresford and the Little Grey Rabbit series by Alison Uttley; then The Ship that Flew; all the "Jill" pony books, and the pony books by the 3 Pullein-Thomson sisters; The Three Jays series by showjumper Pat Smythe; all my brother's Biggles books - and I never found them racist, which they're accused of being today! - Black Beauty .... Monica Dickens' books about Tamsin, Rissa and their ponies, then when I was about 12 my Mum introduced me to Georgette Heyer's wonderful Regency romances."
Back to today - if I had to choose just one, it'd be Tommy the Tugboat, whose author I got completely wrong in 2016; it was Dora Thatcher who wrote him.