What Globalti says, pretty much.
I've got Salsa CroMoto and Nukeproof carbon forks (exactly the same as On One I would imagine). Used them both on the same bike this summer, whilst riding regularly in the Yorkshire Dales. Predominantly, this sort of non technical terrain
Hand on heart, the carbon forks made for all round enjoyable days out, especially when I was unfit at the start of the summer. The trade off is it makes you a bit restraint on the technical rougher descents, picking lines etc, but the climbs are much easier, having a lighter bike and no bob, it makes an overall really enjoyable ride out. I've had no fatigue in my hands or wrists after 4 hour rides (I used to suffer quite badly in the old days and I was 20 years younger). The amount of flex you can see in the forks when you're descending/braking is quite unnerving.
The Salsa CroMoto forks were only used for a few rides to compare, as said above, it was just like going back to my first mountain bike 20 years ago before suspension forks. I don't think the Salsa forks are much like the On One steel forks, so maybe different flex.
I would probably never ride in the Lake District on a fully rigid bike, I go there to get a bit of an adrenaline rush and to ride technical terrain, rigid forks would spoil it for me.