Cyclocross Choice Advice Needed :-)

Not sure what the tyre clearance is on the Diverge but I got the impression it was definitely a tiny bit more road biased than standard adventure or gravel bikes. I could be wrong though.

In a nutshell, good things to have;
Hydro discs
Tubeless wheels
As much tire clearance as poss
A threaded bottom bracket

Have you looked at vitus? Their value for money is off the scale.

I'd also go Boardman, but I'd suggest the CXR instead of the ADV.  http://www.halfords.com/cycling/bikes/road-bikes/boardman-cxr-8-9-cycloc... Slightly different geometry but you can still put slicks on it, and Apex > Tiagra in my opinion (though I have the Tiagra chainset on my bike and it's perfectly fine).

I have been very happy with my Boardman CX Team. At the time it was on offer and already good value, then with British Cycling discount from Halfords it was a screaming bargain. Performance wise I have had no reason to complain, either about the handling or the components (SRAM Rival 1x + Hydro Brakes), apart from a rather annoying creak from the BB. I had similar issue with the BB on my Cannondale Synapse (also pressfit BB) but that was resolved with new bearings and careful installation. I am yet to try the same with the Boardman. I have used mine for two seasons of local CX racing and it has also served well as a commuter with road tyres, mudguards and rack fitted. I would echo what IanEdward said about hydro discs. On that basis alone I would rule out the other three options as decent brakes make a huge difference to the ride experience. I went from cable disc on my previous alloy synapse, to hydro on my carbon one three years ago and the difference was immense. I have however had squealing issues on the Synapse with Shimano hydraulics, yet the SRAM ones on the Boardman have always been silent. The only real negative for me with the Boardman (which I think looks fantastic in the bright orange) would be the reliance upon Halfords staff to build it/maintain it. You may be okay with your local branch, but I have never heard anything good about the ones local to me, so I had my bike delivered to the shop in it's box and assembled it myself, and do all my own maintenance as well. I'd deffo go for the Boardman, it looks great and is very good spec for the money. If you're not already a member, join British Cycling then you'll get 10% off at Halfords which will more than cover your membership cost, meaning you effectively get free 3rd party insurance and legal cover as well.

I think a big factor would be the choice of brakes, I don't mind cable operated discs if they're good, e.g. TRP Spyre, but anything cheaper is a bit disappointing and more awkward to adjust. 

The Boardman has bona-fide Shimano hydraulics which I think is a big plus, I think Shimano disc brakes are the least prone to squeeling in the wet, I'm certainly planning to upgrade my TRP Spyres to Shimano rotors and pads to try and reduce squeeling.

I think the differences between CX bikes and Adventure/Gravel bikes are quite subtle, bizarrely 'gravel' bikes seem to come with more tyre clearance, as everyone is now accepting that 38/40c tyres are a good thing, whereas in CX racing you are still limited to 33c if you really want to compete (UCI rules) so the frames are designed for this, even though this means more clogging with mud! Also I think bottom brackets are a bit lower on Adventure bikes which is nicer on the road but apparently can be an issue if you're trying to pedal in deep muddy ruts or something. 

Oops, I take it back, Diverge has space for 38s too. Looks like a racier position though, good for road but less so for CX, and spec is much lower than the Boardman, probably because of that Future Shok thing in the headtube. 

I'd almost certainly go with the Boardman. I think the geometry of adventure/gravel bikes is better for the road (lower BB) but still fine for CX. I also think the bigger tyre clearance of an adventure/gravel bike is advantageous in CX, I'm still racing an MTB with CX tyres on it, and I think it is a big advantage having that extra mud clearance, the number of mechanicals I see every muddy race makes me wonder if CX bikes are actually a bit badly designed...

Not sure what the tyre clearance is on the Diverge but I got the impression it was definitely a tiny bit more road biased than standard adventure or gravel bikes. I could be wrong though.