I'm sure that Neil Ferguson's model neither is, nor ever was, the only one used in SAGE. It's just the one that grabbed the most media attention, especially in March, and I suppose that (plus Ferguson's complicated personal life) has kept his work in the public eye more than the many other modelling groups. I'd be worried if there were only one group/model in use, for sure.
Just, briefly, on the subject of Brexit, all I would say to that is that it should have been obvious that it was only ever a means to an end for Johnson, that end being personal power. I don't think he envisaged *this* in his first year of leadership, though.