Finally, a topic to get the teeth into for those of us interested in food history.
The PoD would need to be fairly early, keeping things in an Edwardian state of culinary development. Prior to the impact of the World Wars (rationing, social changes to menu structures, decline of the ability to keep servants) British food was very much on a level with any other major Belle Epoque nation. Looking at menus from Germany, France, the USA or Britain, they all follow the same model and feature the same types of foods with relatively minor geographical alterations and sauces.
Escoffier did spend a lot of time in Britain, and there was an appreciation for good food, albeit in amounts that seem excessive to most modern appetites.
Changing the events will leave us with a decent cuisine, but not the superpower level curiously referred to. For that, we need to go back a bit.
The tendency to import French chefs and write menus in French requires a PoD well before 1900.
There is no lack of good ingredients, recipes or cooks as compared to other nations, and British food has not always been portrayed in the derisory light that is rather unjustly applied to it. As referred to, there is great seafood, and this is joined by excellent cheese, butter, beef, mutton, pork, game, hams, pies, puddings, fruit and vegetables. The building blocks have never been absent. Nor is it a matter of how they have been put together, more one of when.
It only really started to get that reputation in the 18th century or so in comparison to the intricacies of French and Italian sauces, and with the changes in the demographic structure of society - the move of large amounts of people to the urban areas from rural parts of Britain directly lead to something of a change in the character and quality of cuisine, as mentioned. We really only start to find the nasty reputation from after WW1.
To change the reputation, we'd need to do either of two things
1.) Reduce the comparative reputation of French food and (much more difficult) the niche of French culture in Britain. That simply creates too many butterflies and is difficult to engineer without going way back into the Middle Ages...and that leads to difficulties in finding sources on culinary history.
So, that leaves us with
2.) Change the British approach to food and food culture from the 1700s or 1800s. This would take an active monarch to encourage the trend among the aristocracy and wealthy. The figure to do it would be the Prince Regent/George IV, who did have Careme over working for him for a while.
The PoD would be for George IV to come to the throne earlier and keep Careme in service longer.
This would need to lead to other chefs being employed, and a proliferation of books and enthusiasm on the topic. Setting up prizes, encouraging restaurant development, competitive banquets between the aristocracy and clubs could possibly flow on from this. The next step would need to be an appreciation of British ingredients leading to the beginnings of a form of appellation d’origine contrôlée for key foodstuffs. Keep this environment going throughout the 19th century, which isn't outlandishly impossible given British economic development, and the stage is set for a bona fide food culture in Britain. That creates the environment where grande cuisine recipes are modified to fit local ingredients, and other recipes created to employ them. Then we enter the 20th century, and hopefully avoid wars, rationing and the unfortunate results.