The Boardman CX bike is on offer at £900 at
Halfords, cheapest drop bar hydraulic bike they have currently.
I have a feeling the tyre clearance is closer to 33mm than the 45mm or even 50mm on the latest Boardman gravel bikes like the ADV 8.9 (which I think was on sale recently but now back to ~£1250).
It massively depends on your local area and your cycling ability, but I wouldn't rule out a Voodoo Bizango (or even the Bizango Pro) for £600/760 respectively, which is a great spec MTB at the sales prices.
If you plan to ride it regularly over the winter then the supplied tyres would better suit if your local roads/tracks get covered in leaf mulch and mud. But come spring, you could fit a pair of 40/45/50mm Caracal Race, which are brilliant for dry riding (I have a 45mm on the rear of my GT Egrade Bolt gravel ebike and might well buy one for the front for next year).