I think the distance between Julian, Anne and dick’s house and George’s was flexible. I’m sure in one book it was a long journey by car, but on another they cycled it one afternoon!
Yes, that's right -- I can't remember which book, but when I was rereading the FF with my child, I remember noticing it, along with Julian, Dick and Anne's variable surname, which switches about between Kirrin and Barnard, despite the fact that we're told in the first book that Quentin is Julian and co's father's brother, so they would share a surname. (Which then pulls up the question of why, unless Aunt Fanny's maiden name is Kirrin, too, all the place names around are called after her family...)
In Five on a Treasure Island, Julian and co live in London and travel in their own car to Kirrin, leaving soon after breakfast and not getting there till six in the evening, but the time time we get to Five Go Off in a Caravan, they live in the country (and have a paddock with a retired pony who used to draw their pony cart, which doesn't suggest they've just moved..?)
(There is also that slightly mysterious reference in the first FF book to Julian, Dick and Anne's father having to meet Aunt Fanny in town 'for a business matter', immediately after which he says 'I don't think things are going to well for them' and proposes that the children go to Kirrin Cottage as PGs. Odd and interesting that Julian and co's father isn't having 'business' dealings with his own brother, but his brother's wife? Selling family heirlooms? Liquidating family assets?)