Colebatch always makes good arguments and has experience to back it up. I've followed this travels on ADV Rider, even remember him here on HUBB in 2000 or 2001 when he was traveling S. America. No idea if HUBB archives go back that far?
Thing to remember about Colebatch is that he goes looking for disastrous routes. Loves to dive into deep waters where a boat would be more appropriate. Most tend to go with more "sensible" routes. ... Does this mean you miss out on something? Maybe, but at least you'll make it through alive and not in a truck. You can ride plenty good stuff without "compromises" to Adventure, IMO.
So, route choice is important ... but it IS nice to not have to worry that your bike is TOO HEAVY to make the planned route. Good research can avoid some of this ... usually.

But surprises en route always can happen.
You also make a few assumptions that are not strictly true. A modern 250cc bike will hold up fine unless brutally canned and held at redline all day, every day. Most are remarkably reliable if given reasonable care/feeding. And even if after 40K miles they need a top end ... it's not the end of the world cost wise.
I'm currently shopping for a Yamaha WR250R. Not really light for a 250 but fairly robust, good suspension, can be made into a decent travel bike. Expensive for a 250 but still half the price of a KTM or Husky 350 ... and FAR more reliable.
Test ridden the WR250 bike couple times, surprised at it's motorway manners. Not bad at all at 65 to 70 mph! I'm coming off Ducati Hyperstrada or DR650 ... both good road bikes.
YES, you could go with a 650 class bike like my
AMAZING DR650 (which you do not get in UK), or find an old XT600 or XR Honda in good shape. Small investment.
But if you intend to really do serious exploring away from known tracks ... then a 250, IMO, is really THE WAY! Rocks, Mud, Deep Sand, Deep Ruts? All much more manageable on the smaller bike over 650 class bikes ... and a world away from the F800GS ... as Walter well illustrates.
I've put 60K miles on my DR650. Not ONE issue. Mostly Mexico, lots of off road in Baja. I've turned back several times in unknown situations on that bike, even though with my set up it's pretty damn good off road.
At just 324 lbs. Dry (147 kg.) ,not bad. Fully fueled & loaded for travel, my DR weighs in at just 405 lbs. (184 kgs.). Not bad in grand scheme ... and good within 650 class comparing FULL TRAVEL WEIGHT.
The KTM 690's are 12 kgs lighter ... but not sure I'd trust one far from home. Too many issues in it's history, even though newer versions are much improved.
(As an aside, most TOO TALL for me)
Many Orange Underpants Army folks point to Noah's ride report ... but never mention all the things that broke on his KTM and how many times things were rebuilt, replaced, repaired.
This from a guy who is an EXPERT at KTM 690's, very good on maintenance and easy on the equipment. So, if you go KTM 690 ... bring lots of money and plan to earn your Master's Degree in 690 KTM service ... bring Spares too!
Or ... you could go 250. Not sure what you can find in UK. If you've got the budget and mechanical skills, you could go for the new KTM 350's or Husky 350's. Very light weight bikes. At around $12,000 USD on the road after Taxes, Fees, they are out of my budget but GREAT performing bikes ... lighter than my purposed WR250R Yamaha.
I will spend about $5000 for a nice, near new WR250R ... and to me, even that is a lot on money. The WR will need about $1200 in additional add ons to be world class. That is still HALF the price of the KTM/Husky, which are hard to find used.
So round and round she goes ... where she stops .... nobody knows.

It really depends on your route choices, your real off road skills and what bike
flips the switch for YOU!
Ride a few, see what you think! HAVE FUN!