I do believe you. I'd question anyone who didn't believe you, given that your Narnia must have been another realm just like Preya is, and they can hardly deny its existence. It's quite fascinating, the way these things work.
I was born in 1915, so I am, in one sense, only a little older than you, and I can imagine just why you would have felt the need to leave London as a child. Are you certain that you'll never see Narnia again? I suppose the time differential makes it so that you'll never see the Narnia that you left. I'm sorry for that.
There is something I have to ask, if you don't mind. This Jadis - you overthrew her because of her cruelty, yes, and not because she was a witch?