Was there any Hawker Trident installed cockpit voice recorder before BEA Flight 548 incident?
It doesn't appear that CVRs were installed in any Tridents before the BAE 548 accident.

The accident report noted that CVRs are installed in some British Aircraft, but stays silent on the Tridents:


  This, at least as far as CVRs is concerned, we do not regard as
  satisfactory. ... They are fitted as standard equipment to all BOAC's 747s and most large United States transport aircraft. 


If any of the Tridents had CVRs, the report would've surely mentioned it. The Chinese Tridents didn't appear to have CVRs long after the accident. In fact, the accident report of a Trident 2E in 1988 notes,


  The  aircraft  was not equipped, neither was it required to be equipped, with a cockpit voice recorder (CVR).


Based on these, it does not appear that any Tridents were fitted with CVRs before the accident.