Other than the short stretch on the A1, and the northbound A167 section after the Honest Lawyer, I don't think the X21 is that demanding of a route. I think there are 2 proper hills on the route (Croxdale and Coundon), the rest of it is just relatively flat 50/60mph roads. Which, is no problem at all for a B5 since long higher speed stretches are where smaller displacement engines are suited. You only really need the 'power' of a larger engine if the route has a lot of hills, as it can generate the torque needed to push the vehicle up without putting as much stress on the engine, whereas a smaller engine would rely on the turbo more to generate that torque.

E400s would be nothing short of overkill for the route, and no doubt use more fuel. From about 2 seconds of googling, the E400s get around 6mpg, the B5s 8mpg, and the StreetDecks 9mpg, obviously this depends on the route, but I'd imagine the fact the B5s have a gearbox that is suited to driving faster than 30mph, unlike the StreetDecks, the B5s probably end up around the same if not a bit better fuel wise than the StreetDecks on the X21.