I think this question is probably all about how long you’re going to keep the bike. Since this thread is about race bikes, I think most who are looking into all these most cutting-ege details are likely to replace/upgrade their race bikes every 3-5 years if they’re racing regularly.
If that’s you, or if you’re testing the waters to see if that’s who you’ll become, then I wouldn’t worry too much about which specific bike you get. Just find something that ticks as many boxes as you can and that has been well cared for and is a good deal.
You’re not at a major disadvantage on 45s just now, and unless you’re already on track to podium in your age group or whatever, then you’re probably going to upgrade the bike long before tire clearance or UDH or other features are realistically holding you back. And if the kind of stuff you’re riding DID mean all that stuff mattered right now then at £2k you’d probably be better off finding a secondhand hardtail MTB and fitting drop bars (and maybe rigid forks).
If it’s going to have mixed use, and especially if you’re not going to have another road bike, then I’d definitely suggest looking for a 2x drivetrain. You’ve got way more versatility in terms of gear ratios in lower budget price ranges that way, and it makes swapping wheelsets etc. more straightforward in my experience. (this isn’t strictly true as there are lots of ways to make 1x work, but the best ranges of gearing etc are often higher end SRAM stuff that can be pretty expensive).
I’m assuming you’re here in the UK based on your stated currency. If so, it’s likely that if/when you upgrade, this purchase will become your winter bike, so mudguard mounts might be worth considering. I’d also say the smaller budget UK brands might be a good shout in this instance for both new and used options: Ribble, Dolan, and Pearson come to mind first. There seem to be a steady stream of barely-used gravel bikes from these brands on the secondhand market and I think both the alloy and carbon versions of all three brands’ gravel bikes can clear 45s and make decent road bikes too.
I’ve just rebuilt my Ribble CGR SL for a 3rd time and am stretching a fifth race season out of it whilst I wait for another year’s worth of new bike releases. It’s a pretty fast gravel bike, a surprisingly good road bike (I swap in the wheels off my Tarmac SL7 to use it as an endurance road bike), and I think I’m going to manage to ride it on 50mm tires this year. You’ll find a bunch of those (and/or the Gravel SL which is the same bike), the Pearson On and On, and the Dolan version on ebay and facebook marketplace at any given time and I think a 105 or GRX build should be under £2k used. Might even get Di2 in budget. I’d even say a Planet X Free Ranger would probably be a good fit.
Now I’ve written that out I’m also wondering if maybe the titanium versions of any of the above bikes might be within reach in the secondhand market? Won’t be as ‘race bike’ in its first incarnation, but if you end up upgrading to more of a speed machine in a few years then an older titanium gravel bike is kind of the perfect winter bike and bikepacking rig…
All the ones you mentioned would be good options too - Grail, Grizl, Exploro. I wonder if you could even find a secondhand Specialized Crux in budget? That would be an incredible buy!