But there was another explanation. When the exiled ANC leadership under Oliver Tambo showed up in London in 1962,  the question was put to the Macmillan government. On the one hand, the ANC was quite clearly under communist control and was involved in an armed ("terrorist") struggle against the South African government, which was friendly to Britain, and had major trade and investment interests in South Africa, not to mention 750,000 British passport holders. To give refuge to such a movement — while it continued to plant bombs in Durban and Pretoria — would be wholly unprecedented. On the other hand, Britain had to take a long view and in the long term, the ANC might well govern South Africa. Would it really be better to expel it and push it further into the arms of Moscow? Moreover, that would align Britain squarely behind the apartheid regime which, for a host of other reasons both at home, in the Third World and in Cold War terms, would be extremely unfortunate. No doubt the Foreign Secretary of the time, Sir Alec Douglas-Home, a strong liberal on apartheid, would have had the decisive view.

What emerged was a secret deal between the ANC-SACP leadership — that is, Tambo and Slovo — and MI5. The SACP and ANC would be allowed to live in London and operate there under MI5 protection but they would not be allowed to carry out any of their military activities there, nor have contact with any terrorist groups, and they would not give the British police trouble of any kind. What that meant was that there would be no public demonstrations against apartheid. At that time, the police had their hands full with CND. This was quite difficult enough: nuclear disarmament was a popular cause, so the protesters could mobilise large numbers. But the anti-apartheid cause was far more popular — Tories, Labour and Liberals were all anti-apartheid — and the cause had huge support from students and Britain's ethnic minorities. Moreover, it aroused even stronger passions than nuclear weapons, so it posed an even greater potential threat to public order. 

This extraordinary deal held for nearly 30 years. ANC and SACP activists came to London from which they fanned out for guerrilla training with the KGB,  the Stasi, other East Europeans and Libya's President Gaddafi. Particularly in the latter case, they collaborated closely with many terrorist organisations which were anathema to London. Often they would then return via London before being infiltrated into South Africa, there to carry out violent acts against what was still a government friendly to Britain. This was greatly resented by the apartheid government and in 1982 it arranged for the ANC's London offices to be both burgled and bombed. However, this was an isolated incident, for South Africa was very strongly warned by Mrs Thatcher against any repetition. A more normal tactic was to get right-wing Tory MPs to make accusations in the Commons that ANC activists in the UK were consorting with the IRA. It was certainly true that the ANC greatly admired the IRA's urban guerrilla expertise and there was some trade in weapons and stolen passports, but its London base was far too unusual and valuable an asset for the ANC to risk dealings with the IRA within Britain. 

The ANC and SACP controlled the Anti-Apartheid Movement and were able to prevent any public demonstrations against apartheid. Yet in 1969, to their fury, the young Liberal Peter Hain launched the "Stop The 70 Tour" campaign. Hain was able to rally sufficient support for his campaign to be a success, though the SACP and ANC tried secretly to sabotage him. The fact that direct action led by a 19-year-old could achieve such a triumph, despite the furious opposition of the groups patrolling that stretch of public opinion, showed the awesome public potential of the anti-apartheid cause.