FlyerTalk Evangelist
Join Date: Feb 2009
Location: From ORK, live LCY
Programs: BA Silver, EI Silver, HH Gold, BW Gold, ABP, Seigneur des Horaires des Mucci
Posts: 14,205
Opt outs are mandatory in the EU, and security clerks are mandatorily required to inform passengers of the possibility to opt out (EU Regulation 1147/2011, annex 1, section 4.1.1.10).
Screening of passengers who opt out in the UK must be conducted in private (Direction to certain aerodrome managers under the Aviation Security Act 1982 relating to security scanners 2015, annex C, section 14).
I have had sight of a letter from the Department for Transport stating that passengers are not required to answer questions about their reason for opting out nor provide personal information. This is not followed in practice and failing to comply results in a D-Y-W-T-F-T. This is from personal and others' reported experience.
The answer to your other questions is SSI and I am unaware of the answers. Even if I were, I would probably be committing an offence under section 58 of the Terrorism Act 2000 to collect, hold, or share such information.