Bear in mind also that unlike commercial transport planes, with light aircraft there are no cockpit voice recorders or flight data recorders which record all control movements and lots of other instrumentation and engine parameters.
In this case it appears the AAIB do not have the have the wreckage either - often in the absense of the above electronic data the wreck may provide clues. As an example the air speed at impact can often be determined by the imprint the speed indicator needle makes on the instrument case dial when the aircraft crashes.
Whether or not the AAIB decide to recover the wreckage will depend on many things. If they suspect some mechanical failure which might impact other aircraft of the same type flying and require urgent fixing on them then obviously the impetus for recovery is more than if they believe the pilot' made an error'. Sadly one factor is simply the cost of such an undertaking to recover the wreckage.
They may seek to recover just certain components - a GPS chips for instance might contain flight logs or position and altitude data which might be recoverable with expert help from the GPS manufacturer.....
So the AAIB may have far less to work with than the mega commercial plane accidents and hence it can be in some circumstances that an explanation of what happened simply cannot be determined with any certainty. In such cases the report can only suggest various possibilities.