I agree with all the rest of antmanb's post, but I have to take issue with this:
I'm not sure on what you are basing the idea that back inside edges are "more difficult" than outside edges. Ask a beginner to hold a back inside edge and back oustide edge and i think you'll find the inside edges are much easier to hold than the outside.
In what situation is this true? If you ask a very beginner to do backward stroking from one foot to the other, they will find it easier to do so on inside than outside edges, but they would be practically flat anyway.
But when you get to actually holding deep edges on curves, back outsides are generally easier.
Somewhere in the learn-to-skate levels skaters are often taught to skate around a hockey circle with back crossovers and then hold a back outside edge (to get used to what will become the landing edge for jumps, although at this point they should do it in both directions). Holding a back inside edge around a circle would be much more challenging at this level.
Skaters usually learn forward outside threes before forward inside ones, but not much before. Once the turn itself is accomplished, how easy is it to hold the exit edge? The back outside edge after the inside three in the preferred direction (i.e., the jump landing edge) is probably the easiest to hold. The back inside edge after the outside three is harder to hold; in beginners usually change feet to an outside edge as soon as possible, or if they're doing a salchow jump after the turn they tend to rush the takeoff rather than holding the back inside edge as long as they should.
The first standard USFSA test that skaters take, either Prepreliminary Moves in the Field or, way back when, the Preliminary figure test, asks for all the edges across the rink on a hockey line. When I took these tests, I definitely found the back insides the hardest to control, and I see it all the time from kids working on the Prepre moves today.
(Also on the US figure tests, back outside eights were on the first test and back insides were on the second test.)
I don't know what the progression of required skills is like in the UK. Does it somehow reflect back inside edges being easier for beginners?