The point is to sell to *private* customers, who then have the advantage of being able to get where they want to go in better than 1/3 the time they can manage currently. There are a small but significant number of wealthy people who would value such a saving of time very highly, so it is a perfectly viable product with currently no competition.
Airlines are not interested in faster aircraft (indeed most run their existing aircraft significantly below their maximum cruising speed), because it comes at the expense of far higher operating costs, and would not attract sufficient mass-market customers who are prepared to pay hugely more expensive ticket prices for the time advantage to make up the money. In any case, for the normal commercial traveller, the time spent on the ground in airports is likely to be longer than the time spent in the air, so shaving 8 hours off the flight time doesn't decrease the total door-to-door travel time by a large percentage. Certainly not enough to justify an additional outlay of several £1000's
Private jets can use small airports that are quicker to get to from where the passengers live, and do not have long queues, onerous security checks, long waiting times for departure or to collect luggage at the destination. Thus the flight time is a far higher percentage of total journey time than it is for us plebs.