You seem to be set on buying one brand new for £600, i would go for the nordictrack if thats what your going to do.
But unless you need to do running for a specific reason, it could be cheaper to do another form of cardio like skipping which would cost like £5 for a plastic speed rope off ebay/amazon or rowing, you could get a really good rower for £600.
My Treadmill's RRP is like £1500+ and it feels really cheap compared to what I'm used to at the gym (technogym) but it's a lot better than cheapy argos equipment.
I wouldn't put much faith in a £600 RRP one tbh a treadmill is one of the most expensive pieces of cardio equipment you can buy, e.g. technogym's top end model is £16,000 compare that with the best rower on the planet a brand new concept rower is only just over £1K.
1/16th of the price for a like to like comparison, just to give you a comparison on how expensive it is for a top end treadmill compared to a top end rower.
End of the day if you want a treadmill buy one, but i find cheap treadmills are just very unpleasant to use, i can just barely stand my £1500 one, the speeding up and slowing down of it is horrific as is the feel of it below me and the smoothness of the belt, but I am used to running on a technogym.
It's a bit like driving a rolls royce your whole life and now having a focus, the model your looking at would be like a fiesta in comparison. It will do the job but it won't be as smooth or nice an experience