Considering I bought my new PS4 in November for £220 with the Uncharted Collection, I don't see how having a slim going for £249 or above will significantly increase Xbone sales.
Clearly, it's addressing many of the image issues that the original Xbone has suffered, but with a new, more powerful console on the horizon that is apparently BC, I would have thought taking a financial hit on the slim would be a great way to hook people back into the Xbox eco-system ready for Scorpio. Sell it for £199 and I can see plenty jumping on it, even just as a handy 2nd console to own.
Then, when Scorpio releases, people might jump because the cheap price means they don't feel burned by their slim purchase and they have built up a library they want to carry over/upgrade.
I just don't see £249 as appealing enough as a price to convince those who are yet to get a new console to opt for the Xbox instead of a PS4, or for those PS4 owners to buy on a whim as an exclusives box.