The Hawker Siddley Trident was a little before my time in British Airways but many of the Captains that I flew with, when I first started, had operated it in the 60's and 70's.
The use of airborne reverse thrust was certified, and often used, achieving incredible descent rates of up to 10,000 ft/min.
For those that have never flown big jets, one of the surprises, is how difficult it can be, from high level, to get rid of excess altitude. There is a lot of inertia and potential energy in a big jet going at Mach .8 or so at high altitude and getting rid of that in a way that suits Air Traffic Control can be mighty difficult.
Incidentaly, Concorde was also certified to use Reverse Idle in the air, to assist in reducing speed from supersonic cruise.