I can't remember any mention of rationing at the time of Peter's Room - there was no mention of it when Mrs Marlow was going to take Ginny into Colebridge to buy a dress for the party. She did talk about the price, however.
As for Catkin, I thought someone (Rowan I think) said clearly to Lawrie that she shouldn't get her hopes up, if Ginny outgrew Catkin, she'd sell her to buy a horse...