There would not be any reason for them to go any faster than Mach 2.0 because that´s the limit for aluminium, heatwise, i.e. the Brits designed an airliner restricted by the material, and found out that airspeed greater than M 2.0 (approx) resulted in too much friction heat thus weakening the ship.
The Americans (Boeing SST R.I.P) decided to go faster. The rest is history, Boeing quit their plans after realizing their fundamental error and millions squandered. Of course there are better materials now but they are probably off limits financially for mass produced airliner. So, I´d presume we´d be flying M 2.0 again.
