Looks like fun day on the bike, but the warnings about what you’re required to carry are kind of extreme… I guess the climate is comfortable enough that some people might take the wrong risks and get caught with a bad mechanical…
My $0.02: If you want a large and clean fit, you’ll likely have to pay for a “custom” from a company. https://roguepanda.com/custom-frame-bags/ was the first one that popped up in my googles. Your googles might have different results in the UK.
For a 200km day (with nutrition/hydration resupply), and no overnight, you might get away with a half bag from any number of companies. Regular tool kit in regular seat pack covers all tools but the pump… emergency blanket, rainshell, and dry shirt should fit in even a smaller top tube bag, as long as you’re not worried about wrinkles. You can probably wedge some gels/bars into the bag and pocket carry an additional banana and gels.
Actually, Canyon sells a Topeak frame bag for the Grail, which has a similar, if less pronounced angle at the seattube/toptube. I’d suggest going to a bike shop (kind of the opposite of Canyon’s business model) and test fitting some bags they have in stock. If they have one that fits, buy it.
So you need a half frame bag our want full? You’re not camping, but you are carrying food. It’s in Sept so you don’t need a lot of clothes.
I use Apidura’s 2.5L half (long and deep) Racing bag for general rides. Right now, it’s got a pair (dress type) merinio socks, glove liners, and my flat kit (soda can sized, a little narrower) and it’s full. I can probably cram a soft pretzel in there if it looks good for later. I get a lot of use out of it, so money spent here is well spent.
Id recommend getting one that’s not any wider than the one above (6cm) as well as one with some basic structure. The structure will reduce swing, so you don’t need both ends tired (if it’s a half bag). Silicone straps help too. You also don’t need to buckle both rearward straps, just maybe a lower one on a long one. If you’re on a 56 or 58 or more frame, I’d go with one that’s deeper in front, rather than a long shallower one. You might have a kiss a few frogs here, so buy from places you can return it.
Side exit bottle cages are a much (Elite Cannibal work well). you probably want the back of the seat open for a TT bottle setup… OR a traditional RACK might be a good idea over the rear wheels.
According to his, you’ve got about 380mm to play with before the funk begins
So for a shorter frame bag with room for water bottles over 3L …