Thanks so much for the advice everyone!
After a long chat and a lot of emails with Oxford Bike Works I've decided to go down a slightly different avenue. I've gradually realised what I want are two different bikes - a modern, utilitarian world tourer that can handle anything including long tours and last decades (I admit 'forever' is a slight exaggeration, but you catch my drift

) , and a lighter, faster bikepacking style bike for shorter weekend trips and multiday rides with my partner around Europe going by train.
I spoke to Oxford Bike Works and considered building a more expensive custom bike that fit both of these specs. Yet I gradually realised it wouldnt be worth the extra money and time, and that no bike could ever do both so effectively.
So, instead I'm going to get an Oxford Bike Works Disc Expedition built with all the bells and whistles I have long wanted and been unable to add to my Dawes Ultra Galaxy. It completely fits the original spec I wanted (disc brakes, dynamo with lights and usb charger, 700cc wheels with 35c marathons, flat bars, mudguards, kick stand). The only difference is that its an all-reliable, heavier beast of a tourer that will not in any way resemble the speedy bike I originally wanted, simply because that just can't really be done. I'm also happy to leave the more technical details in their hands knowing their solid reputation for building indestructible touring machines.
With that said, my current Dawes Ultra Galaxy does still fit the light tourer spec I want pretty well, and I'm likely going to tear the back rack from it and use that for shorter, faster rides.
Once again, thanks for the fantastic advice and I would be more than happy to share some pictures when it is all built come Spring!