The Nationality Act 1948 allowed for former subjects of the British Empire to enter Britain, and merely required one to arrive here and register. It was not conceived to deal with the much larger post war migration of labourers.
The Commonwealth Immigrants Act 1962 introduced a voucher system in an attempt to align employment needs with immigration supply.
1964 saw the end of vouchers being issued for unskilled workers, with them being restricted to professionals, skilled workers and those coming to work in employment in which there was an acknowledged shortage, such as the medical and transport industries.
In 1965, a new restriction was brought in to limit the number of vouchers that could be issued to any one country. A maximum of 15% was set - seen by many as an attempt to limit immigration from the Indian Subcontinent.
The Commonwealth Immigrants Act 1968 withdrew automatic right of admission to citizens of the Commonwealth but favoured those who had a British-born parent or grandparent - a move seen as advantaging members of the Old Commonwealth countries of Australia, Canada and New Zealand. When independence was granted to Kenya (1963), people resident there with Asian or British heritage were assured that they would retain their British citizenship and thus have right of entry into Britain. This assurance was by the 1968 Act which deterred an influx of Kenyan Asians with full British passports from re-settling in Britain. However those who did come often made rapid progress in establishing themselves in business or professions.
The Immigration Act 1971 introduced the notion of "patrials", who would have right of abode in Britain. All non-patrials needed permission to enter the country, whilst all those admitted under earlier legislation retained the right to bring to re-settle here their close relatives. People from Old Commonwealth countries were more likely to have a UK-born parent or grandparent, so the Act was held to strike hardest against members of New Commonwealth countries, who were reduced to the same status as "aliens", namely that they needed permission to enter the country and needed an annual work permit.
In the Ugandan crisis of 1972 President Idi Amin gave notice to the "Ugandan Asians" to leave the country. Public opinion would not tolerate a hard line being taken with the bearers of British passports and so extra entry permits were issued in this exceptional case. From 18 September to 7 November 1972, 28,000 men, women and children classed as Ugandan Asians were allowed entry into Britain.