Commonwealth citizens became subject to immigration control in 1962, but before 1 January 1973 (the date the Immigration Act 1971 came into force) they could not be deported once they’d been here for five years. This exemption from deportation was preserved by s 7(1)(b) Immigration Act 1971 and is still there – for those who were in the UK lawfully on 1 January 1973 and have been here for the past five years.
In 2000, overstayers were no longer subject to deportation, as they used to be but to administrative removal instead This was implemented by s10 of the Immigration and Asylum Act 1999. The exemption for long-resident Commonwealth citizens was continued, under an amendment hidden in Schedule 14, para 46 of the Act. It was updated by s 75 of the Nationality, Immigration and Asylum Act, which added a subsection (10) to s10 of the 1999 Act.
So Commonwealth citizens detained as overstayers for deportation (until October 2000, when the 1999 Act came into force) or for administrative removal (from October 2000 onwards) could not be deported or removed provided they could demonstrate that they were in the UK with leave on 1 January 1973 and had been here for the five years immediately prior to the decision.
The exemption was abolished in October 2014, when s 10 of the 1999 Act was replaced (see s1 Immigration Act 2014). Sec 75(4) of the 2002 Act has been repealed (see Sch 9 para 7 Immigration Act 2014). Also, current Home Office guidance on enforcement, including removal of overstayers, makes no reference to the exemption.
So now, Commonwealth citizens here since before 1 January 1973 who cannot prove they are in the UK lawfully can be removed as overstayers