The British knew if they did this it would create a conflict that they would be drawn into. By the 1960s the might of the British Empire had ended and so any conflict involving Britain against any rebellious state would most likely result in their loss and embarrassment.

@andrefalksmen1264: The unoffical policy of the Wilson government was not to invade owing to the "kith and kin" rationale i.e. it would be unpopular among the British people and units within the British armed forces. A contingency plan for invasion was made by British defence chiefs who warned of the risks involved in attacking a landlocked country. The "kith and kin" doctrine is of course in stark contrast to recent bloody interventions in West Asia (Iraq and Afghanistan overtly and Syria and Libya covertly).

@adeyinkamakinde6164 another example of the degradation of British leadership in the 20th century. They missed a golden opportunity to secure themselves and appear virtuous. By capturing South Africa and Rhodesia, and administering them on the same basis as Hong kong, they could have claimed to be defending human rights and their treaty obligations, and sovereignty is Rhodesia was a rebellious colony, while at the same time garnering control over the natural resources of South Africa and rhodesia. The gold alone from South Africa would have bolstered Britain's reserves and prevented its repeated evaluations which eventually led to the breakup of the Sterling block.

A settler colony which had no business being there. They are lucky that the native population didn't indiscriminately eliminate them.