EAP
Neither interviews with the AAIB nor the police are taken "under oath" as you put it. Evidence to be used in court is not taken under oath in either police or HSE investigations.
Witness statements taken during a police or HSE investigation contain a declaration as to the truth and interviews are conducted under caution.
Live evidence in court is given under oath.
Since Rogers v Hoyle, any person being interviewed by the AAIB where there is a suspicion of negligence on their part would be well advised to seek expert legal advice before doing so and should without doubt be legally represented in any police or CAA investigation by a solicitor who is a specialist in this field.
The position with AAIB reports is that at present they are admissible in civil proceedings but not in criminal proceedings. An interview with a pilot is annex 13 material and not in itself to be disclosed without a court order. Such a statement has not as yet been ruled admissible. The difficulty that arises is that summaries or extracts of the interview often appear in AAIB reports and the contents of the interview may have some bearing on the opinions formed and the conclusions reached.