Treadmills aren't great general running training. Even with the incline up the motion is different. A treadmill is moving, so imparts kinetic energy to your foot while it is touching it: it carries your foot backwards. You aren't pushing yourself forwards, your legs are just stabilising your body while it hangs in midair.What is the terrain at Pirbright as that is where I will be going for my assessment centre. When I tun on a treadmill (with the incline turned up to 1.0) I can run the 2k in 8mins 39 secs. When I run outside through Southampton common, which has a large incline at the end I struggle to do it in 11mins. Any advice?
The ground, on the other hand, is not moving (perceptibly), so 100% of the kinetic energy comes from you hitting the ground and then pushing up and off from it. This uses your muscles differently, so gets them fit differently. You have to do all the work to move yourself forwards.
Inclines, whether on a treadmill or the ground, require you to push up more because you have to lift your feet and body higher with each step. This makes it harder work, and works your "uphill" muscles (quads and calves), but doesn't solve the basic issue of treadmills carrying your feet backwards for you.
Treadmills are useful if you can't run outside, or for things like sprints and intervals which need to be accurately timed and paced, but those are mostly about building your cardiovascular fitness. To improve your outdoor 2k run time, you need to run outdoors.