Advance fares seem to waver between being TOC▸ specific and system-wide. It doesn't seem to have been decided whether the ambition is that they're intended for a TOC network or between any two stations, perhaps on a 'Core train and connections' basis.
They're TOC-specific - my understanding is that whichever TOC has responsibility for setting the flexible fares on a journey also sets the Advance fares, where they are available. It is TOC choice whether or not to offer Advances for any flows that they price. Also, their decision whether to offer any 'any connections' to stations (on other TOCs networks) that branch off their main routes.
Inspired to post this having attempted to set up a couple of TOC ticket availability alerts, and there seem to be several mechanisms on the go for *that*. LNER» only allow alerts for tickets involving journeys by LNER trains. Crosscountry... odd. Go to their ticket alert page and set 'Bath Spa' as your starting station and the drop down in the destination field then only contains a single station 'York'...
Left over from when one of their very early morning departures was from Bath I think pre-Covid.
Also inspired by the perception that, when bought as an add on, GWR▸ have tended to make the Bristol to Bath length of the GWR main line one of the most expensive in the country. (OK, probably justified by the engineering works...)
One of their most popular pieces of track, it's no wonder that they try & get as much revenue as possible from it. Always remember that the sole purpose of selling Advances is to fill seats that otherwise wouldn't be filled - and with Bath - Bristol being popular, they don't have to sell that many. AS an example, it is often cheaper to buy two off-peak day returns from London to Bristol (splitting at Didcot) than an Advance from Paddington all the way. And you get off-peak flexibility too as a bonus.
I was also inspired by recalling the famous 'Advance ticket 1st class' bargain Alness to Bath Spa 6 years ago for around £100. Current attainable price seems to be around £267, some of the increase may or may not be down to the GWR leg involving a peak time departure from Paddington, but that segues into the observation that for long distance travel, some part of the trip is always going to be in the peak and if that bumps up the entire fare to an 'Anytime' fare then that goes a long way to explain the average journey length and passenger churn on Crosscountry, whose strapline could be 'Your national local train'.
My bet is that is probably priced by
XC▸ , who are renowned for having pushed up the Advance fares big time over the past 5 or so years at least.