We're planning a drive from London to France when my baby is 1 year old. She is however on a low centile and probably won't weigh the required 9kg then, more like 8.5 kg. She's not awfully short though, just slenderish.
But she's really outgrown her MaxiCosi and her legs are bunched up and her head is scraping the back of my seat (erm, should explain that the interior of the sportscar is quite small...)
Her development in terms of holding her head, sitting, pulling up to stand, cruising is early or on target, so her small size hasn't affected any of this.
I'm unsure of whether to move her to a forward facer for this journey or just persist with the rear one (I understand some advise continuing with a rear seat until much older, but our car won't fit a rear facer for an older child.