I'm not sure that clip confirms that SAGE, as a body, were ever in favour of herd immunity, it would be news to me if they were. My recollection is that, as shown by the video you linked, Valance, and maybe Whitty, spoke in favour of herd immunity for a short period of possibly as little as two or three days before a decision was made that the number of deaths, as predicted by modelling, was too high and another approach was required. The situation was so fast moving that I'm not sure SAGE would have been able to convene a meeting where all members could agree with what Valance and others were saying for that very short period before the herd mentality approach was abandoned.