And London to LA or NY are definitely routes with that demographic.

The last all-business-class flight that served London to NY (BA1/BA2) was shut down in 2020.

It flew from LCY to JFK with a stop at Shannon in Ireland for quick refuelling and US Immigration preclearance (so you landed in NY like it was a domestic flight). It was an Airbus A318 in a 32 (if I remember correctly) all business class configuration.

It stopped in Shannon to refuel, since LCY is too short for commercial transatlantic hardware to take off with enough fuel. The customs clearance in Ireland was a smart way to take advantage of the stop to make things less painful in New York.

Brexit probably killed it

Nothing to do with Brexit at all, since Brexit had 0 impact on travel between the UK and the US, or on air travel between the UK and Ireland for that matter, because neither of those countries were in the Schengen area.

Covid killed the flight.

Brexit killed England as a financial center. That also impacted travel to the usa

The big airlines have business class and first class. You can pay more for a nicer experience now if you want now, hard to see being able to run a first-class only airline.

A few exist, e.g. https://en.wikipedia.org/wiki/La_Compagnie