Without a NP victory it is possible that Southern and Northern Rhodesia could join the union. After the war the legislative assemblies in both colonies debated the pros and cons of joining the union, and it seems that the verdict was that the colonies must either be joined together or join the Union of South Africa. The British Parliament was debating the issue as late as 1952 on whether to federate the two colonies together or have them become part of the Union. In the end, they both entered a federation together and were forced to take on Nyasaland as part of the deal.
By 1946, both colonies had a large proportion of their European populations born in South Africa (over 30% of the total), and around 15% of Southern Rhodesian whites were of Afrikaner descent, in Northern Rhodesia that number was closer to 20%. Even with the post-war immigration from Britain, South African-born whites still outnumbered British-born whites in both territories according to the 1951 census. Even as late as 1969, South African born whites were nearly equal to British born whites in Rhodesia.
As for Bechuanaland, it might be amalgamated as a separate territory, whereas the Union would govern it much as Australia governed Papua and New Zealand governed Western Samoa and Nauru. Basutoland and Swaziland could possibly fit into that mould too, but only if the respective rulers agree to transferral of power from Britain to the Union, which is unlikely.
Barotesland would probably be separated from Northern Rhodesia, although a part of the protectorate, it was governed separately from the rest of the territory, even during the federation years. Europeans were not allowed to enter the territory without consent of the British Resident and the ruler there would not acquiesce to amalgamation. This could force Britain into creating a completely separate protectorate in the guise of the High Commission Territories. Similarly, North-Eastern Rhodesia would most likely be incorporated into Nyasaland protectorate, as this was proposed in 1952-1953 should Northern Rhodesia become part of Southern Rhodesia or South Africa. Northern Rhodesia would consist of the central part of the country along with the Copperbelt, where most of the Europeans, and one-third of its African population lived.
As for Southwest Africa, the Union may try to incorporate, but they would have to do this early on. The lack of apartheid may make it more palatable to the UN (even though South Africa never transferred the League Mandate over to the UN). Regarding debates on the subject in 1946, it seemed that even non-western countries at the time such as Mexico and Syria were willing to go along with the incorporation. This of course would radically change by the 1960s. As the European population in the territory was overwhelmingly Afrikaner (60%) and German (33%), it could be a way of mollifying Afrikaner opposition to the Rhodesias joining.
Finally, if the UP remains in power they keep their policy of assisting and encouraging large numbers of Europeans immigrants to settle in the country, only helping their cause. Once the NP won, they sold the South African government's fleet of liners to Australia and limited European immigration, while trying to attract more Dutch immigrants. The result was that immigration tumbled in 1949, and it was only in 1961 that the NP decided that it needed more Europeans.
