The thing you might not have taken into account is the actual experience of the teacher who would like to introduce students to programming. I have no experience with the British school system, but I did work for IT in a K-12 U.S. school system not too long ago, so I think I have something to say about this.
Where I worked, students' computers were heavily locked down Windows machines running a restricted set of software. Because of the machines' age, the bad third-party GPO-wannabe software that the school district used to manage the systems, and various virus infections, these computers were not the friendliest things to teachers and students – and both groups were perpetually scared to death of "messing up" the computers and getting in trouble. In reality, these PCs were used primarily as overcomplicated interfaces to various bits of flash- and web-based educational software, and anything else was deemed too troublesome.
The point is that between the technical deficiencies and the bureaucratic ones, getting school IT to allow students to run a new type of program and then support it can frankly be a nightmare. You say these computers are capable of running Python, and this is true in the strictest sense, but in reality it's just not going to happen when half of the admins don't even know what Python is, and the other half are too scared of deploying a new, "nonstandard" interpreter.
And if that's how IT feels about the prospect, just think of how frightening it looks to the teachers.
Now contrast that with using something like the Raspberry PI. You can program without messing up your "real" computer! No IT support required, you can reset it to factory configuration in a heartbeat, and even if you do manage to physically break it somehow... hey, it was only $25. Perhaps most importantly, you can write a grant proposal to get a classroom full of them without having to go through the IT department. The Raspberry PI, or something like it, is the programming tool that teachers will be able to use in practice.