On the subject of a non standard round, it is a risk, i am taking it for two reasons The POD is the Thai order so no need for 303, also the order is big enough for a brigade sized deployment, thats not so small that supply is impossible. Also given the real problems with rifles in the early days of the war i think that a rifle that senior officers like, and now the secretary of state for war supports is going to get used. In addition the rifles belong to the battalion, not the army, the London Scottish was rich, the annual subscription in 1914 was 10 pounds, they could easily afford it. The 4th Cameron’s were equipped with the long lee Enfield at the start of the war they lost them and got arisakas then they got the SMLE in France. Tjhe 280 is not as hot as the 276, its more like a rimless 303 necked down to 7mm
There may be enough rifles produced for a Brigade, but AIUI the London Scottish is only a battalion at this point. And the battalion may own the rifles, and maybe even the ammunition, but the War office owns the logistics train, and it is this that will determine if they allow them to be shipped to France.
To put in context the Logistics headache that this is going to entail, The .280 is going to need its own packing crate. It will need to be separately labelled, loaded and stored from the factory to the port (on civilian railroads). Since the volume is not large or steady enough to justify keeping large numbers of rail cars available to haul it they will have to request one, from an already overloaded system, whenever they need to send out a shipment. They will need to be separately unloaded and packed on ships crossing the channel (and I would not blame the stevadores for mixing ammo crates with other ammo crates, regardless of what is stenciled on the box), and stored separately in the warehouses on the other side. They then have to not be picked up the first time someone is looking to load a train with ammo, but be held for a specific train, going to a specific part of the front, and not have masses of other ammunition stacked in front of it when that train needs to be loaded. Once it gets to the rail head it will be stored on the ground until the transport units pick it up. Throughout the war there was a continuous shortage of wood on which to stack supplies at the railheads. That means that the .280 may well be directly on the ground absorbing moisture. And since it is a smaller pile a greater percentage of the total is going to be soaked. Then the transport companies (who may be attached to a specific unit or may not, depending on the location and units involved) need to pick up the right crates, and carry them to the right battalion in the area they serve. And by that time the battalion and the brigade it is attached to may have moved to a different part of the front, and now cannot use the ammo in stock there, and will need to somehow get the stuff stored at their old location to their new one.
Basically there is a very good reason why logistical concerns often trumped technical ones when it came to small arms adoption.
I did think hard about sticking with 303, but the army already knew it wasn’t ideal, the 276 wasnt either, they still want to be able to kill horses so a 4.5mm 3200fps round wasnt on the cards either. Now that Kitchener is dead the Territorials are going to go over in numbers so the need for workable rifles becomes even more acute.
Once war breaks out, not being ideal is irrelevant, as you have lots of them and lots of guns that fire them. You are not likely to change to a new calibre for your frontline troops in the middle of a war of attrition. That kind of thing was one of the major problems during the Crimean War that lead to the death of the Ordinance Board as an independent and powerful organization within the army. What's more, the .303 was plenty good enough for WW1. In fact it was probably better suited than the .276 that they were looking at.
The supply of material for the probably expanded territorials is likely to come from the same pool that IOTL equipped Kitchener's New Army. So the conditions that informed decisions for supply IOTL are likely to be pretty similar ITTL.
If I may, you can get the FQH in service with the British Army from the point you are at without really needing to create the havoc that a non-standard round would cause. Churchill is more or less King of the Military ITTL and he has seen and liked the FQH in .280. It would not take a huge push from him to get some ordered in .303, especially since the FQH was made in .303 IOTL. IOTL the British had evaluated something like 20 automatic rifle designs from 1899 up to 1914, including several iterations of the FQH. The plan was never for general adoption but to issue them to a certain number of troops, kind of similar to the later use of SMG's. This philosophy would dovetail with the perceived (though possibly erroneous) need for a "Walking fire" weapon to help keep the enemy suppressed while crossing no-mans-land in the early war. So ITTL Churchill could place a large order for .303 variants with either box or drum magazines for this purpose. Then once the rifles are at the front and proving themselves the call for greater production as a general use rifle is made, and post war the .280 round is already there ready for adoption if the British so desire.
Nonetheless, as I said, I am enjoying the timeline either way.