'whence' question upon British citizen's return

Dear Madam or Sir,

Some British citizens reported that, upon returning from abroad at
London Heathrow Airport, they were asked at the passport control by
UK Border Agency (UKBA) officials, in addition to checking their
passports, from whence they had returned. In comparison, British
citizens, as citizens of a European Union member state, have
reported that no such question was asked at passport controls in
other EU countries (for example, at the Helsinki-Vantaa Airport).
In connection with this, would you please provide the following
information:

1 What criterion is used by UKBA officials at passport control to
determine whether to ask British citizens whence they had returned?

2 Since British citizens have the right of abode in, and return to,
the United Kingdom, what is the purpose of such a 'whence' question
at passport control?

3 What information is collected and recorded by UKBA officials at
passport control about the British citizen upon return? In
particular,

3.1 What information from the passport is collected and recorded?

3.2 Whether the answer to the 'whence' question is recorded?

Thank you for your attention.

Sincerely,

Dr Kaihsu Tai

Dear Ms Ho,

Many thanks again to you and your colleagues for the timely and
clear response.

Sincerely,

Kaihsu Tai