Google "UK iPad shortage" and you'll see plenty of articles showing Apple cut back on production of iPads to concentrate on the newest iPhone models to meet demand. Plenty of places were low on stock before Xmas, and we know the Inghams like to leave things to the last minute, and put in as little effort as possible.
As for not wanting top of the range, these people are still trying to convince the world they're set for life, they're not going to settle for an older model. Plus Chris kept banging on about the iPhones being the latest version.
I still think it's possible they left it too late due to their European tour of Lidl supermarkets, and couldn't be bothered to make the effort to hunt one down for her.