Reality check. Let’s compare to France and it’s well developed high speed rail network.
Edinburgh and Bristol are (on one list at least, opinion varies) the 5th and 9th largest cities in the U.K.
In France the 5th and 9th largest cities (on one list at least, opinion varies), are Nice and Bordeaux.
Between the former pair, around 320 miles as the crow flies, there are 5 daily flights, and an hourly train service that takes around 6 1/2 hours
Between the latter pair, around 400 miles as the crow flies, there are 3 daily flights and no direct trains. If you do want to use the train, it’s 9 hours minimum and a connection in Marseille - there’s 6 direct trains from Marseille, but the connections from Nice generally aren’t great. It’s not much longer to go via Paris, nearly twice the distance.
I could have picked Strasbourg and Lille, or Toulouse and Nantes, or any number of large city pairs that aren’t on a direct route to Paris. Regional rail connectivity in France is generally much worse than in the U.K., unless you are on a route to/from Paris, in which case it is usually quicker but less frequent.