Renounce BC and retain right of abode

[smalldog]

Before anyone says it, I know that renouncing British citizenship generally means losing right of abode. However it seems that the law may allow it in a certain set of circumstances, although the last post I can find about it is from 2006 (here).

I've quoted the relevant bits of the law below. My specific circumstances: born in UK before BNA 1981; mother and maternal grandparents born in the UK; exploring applying for a Commonwealth citizenship (Singapore) that would require me to renounce existing citizenships. Would appreciate hearing if anyone has tried this out.

Immigration Act 1971 (current)
2 (1) A person is under this Act to have the right of abode in the United Kingdom if—
(a) he is a British citizen; or
(b) he is a Commonwealth citizen who—
(i) immediately before the commencement of the British Nationality Act 1981 was a Commonwealth citizen having the right of abode in the United Kingdom by virtue of section 2(1)(d) or section 2(2) of this Act as then in force; and
(ii) has not ceased to be a Commonwealth citizen in the meanwhile.

British Nationality Act 1981 (original)
2 (1) (d) he is a Commonwealth citizen born to or legally adopted by a parent who at the time of the birth or adoption had citizenship of the United Kingdom and Colonies by his birth in the United Kingdom or in any of the Islands.