The Sea Vixen has a RAT, which AFAIR was primarily for hydraulic services, particularly the flight controls. Daily check before / after flight with automatic deployment as normal hydraulic pressure decayed.
The undercarriage had an emergency lowering selection on the normal gear handle, being either a segregated hydraulic supply or a dedicated reserve.
Thus a gear up landing would be the result of very unusual circumstances. The gear could have suffered a mechanical jam, but all three at once - selection system.
The hydraulic flight controls appeared to be working; thus some power may have been available - not a complete loss of fluid? Flaps up perhaps a recommendation for a gear up landing; wing tank retention appears to have been a good decision.Tail bumpers are sturdy metal blocks which could be used during aerodynamic braking, but in this instance they may have minimise rear fuselage damage.
Canopy jettison might have been a well considered alternative because the operating system is electric, and presumably the plan was to select all power off after touchdown. The canopy elect and a manual backup / emergency unlock IMHO were not very reliable.
All of the above subject to failing memory of a low hours, love / hate relationship with the aircraft - involving many elect / fuel pump failures, undercarriage, canopy, and hook problems; fortunately only operated the aircraft feet dry.