Note: the BAe 146 / Avro does have wing spoilers.
Air brakes are in most cases mounted directly on the fuselage. Forward mounted air brakes would disrupt the airflow to the wings and engines, so the designers are left with three choices: above, under, or at the rear / on the sides.
Under, like on the Aero L-39, works well1 if the air brakes aren't big. Above is usually if all else fails, like on the F/A-18 Hornet and F-15.
Rear-mounted—inline with the center-of-mass—is the most convenient, aerodynamically and for maintenance. Like the Avro, F-16, or F-86.
Now, as to why the Avro—as most military jet aircraft and few jetliners—has it:
Lack of thrust reversal and steep approaches
The Avro RJ lacks thrust reversers "due to their perceived reduced effectiveness in anticipated conditions." The reduced effectiveness is because of the low landing speed and the closeness of the engines to each other on each side. Using reverse at low speed risks FOD ingestion and stalling of the nearby engine.
It reasoned that since the
146 would have a touchdown speed
of only 90 kt, and because reversers
are not normally used below about
60 kt, their effectiveness would be
limited. Reverse thrust also would
have subjected each engine to an
Maximum descent rate achievable
with the airbrakes is 7,000 ft/min and
at speeds of less than 250 kt and below 10,000 ft, there is still 4,000 ft/min available. By comparison, a
typical airliner descent rate on the
approach is around 2,000 ft/min. BAe
146 operators may also use airbrakes
during the landing roll.
One advantage that came out of having air brakes is the ability to fly steep approaches into airports like London City.
[For] several years the BAe 146 was the only conventional jet aircraft capable of flying from London City Airport.
Because of the Avro's high thrust-to-weight ratio, having air brakes deployed on approach makes it easier to control the speed. The thrust-to-weight ratio for the RJ100 is 0.28:1, compared to 0.16:1 for the comparable Boeing 717. It's even higher for the smaller RJ's.
Having a high thrust-to-weight ratio also allows operations from short runways and runways within cities, as the airplane can climb more steeply, thereby reducing the noise impact and also allows high payload.
Air brakes differ from spoilers in that air brakes are designed to increase drag while making little change to lift, whereas spoilers reduce the lift-to-drag ratio and require a higher angle of attack to maintain lift, resulting in a higher stall speed.
The Avro still sports wing spoilers as shown above, those also help in braking as they transfer more weight to the wheels, and help reduce the likelihood of a bounced landing.
1 Belly-mounted air brakes on the L-39 help the pilot configure the plane for landing. Half-flaps + air brakes require the same trim/power as full-flaps and landing gear down (hence the under positioning). The pilot would use the air brakes to slow down, put down half the flaps, and trim the aircraft. The pilot can then on final approach put down the landing gear and full flaps, retract the air brakes, all without having to re-trim or change the engine power setting (which is narrow-ranged on the L-39).
Related: Why does this regional jet have its air brakes wide open before touchdown?