ASWDU's records are at Kew - the reports are in series AIR 65 and their ORBs in AIR 29.
If you search the catalogue
The National Archives | Discovery Advanced Search Form for the trial number and specifiy the reference AIR 65, it will probably return reports with titles sufficient to give you an idea of the trial subject. For example, searching for 300 retrieves AIR 65/438, which has the title "Detection of submerged submarines by aircraft (II). Trial 300: A proposed tactical use of the passive directional radio sonobuoy"