If GWR didn't want these tickets, they'd just tell Greater Anglia to remove them. GWR would have had to agree to the initial creation of these fares, and reach agreement with Greater Anglia as to how the revenue is split, in addition to specifying quota. The quota will just be set low against each of the tiers. Ultimately there's no need to play games around blocking availablity.
I realise that you are talking from an informed position, but to the outside view the observation that there is practically no availablity (other than some nights on the Night Riviera and limited other services) of advances from Penzance to Norwich suggests that the quota may be set to a very low number.
I note that on 18 July, looking at GWR's booking engine, an advance can be had for £56.50 from Penzance to Norwich on the 0605 departure, changing in London and using either the Elizabeth Line or London Underground to get between Paddington and Liverpool Street, but not on any other service.
This incidentally undercuts the £60.50 GWR want for Penzance to Paddington for the same service. Admittedly, the 0605 is the cheapest departure of that day for Penzance to Paddington advances, but £56.50 isn't the most expensive of the Greater Anglia priced advances for the flow. The advance price steps are £34.00, £40.00, £47.50, £56.50, £65.00 and £81.00.
If, as you say, GWR have to agree to Greater Anglia's pricing, then why don't they just influence Greater Anglia to hike the prices and release a quota on more services? I would understand the position more if GWR had sold out of all their own advances for the route, but that isn't the case.
