Not all support bubbles involve children. Ours involves another single adult, living alone and working from home. The COVID risk is minimal.
I have always thought and still do that while the rules are there and should be followed. They are a broad brush approach to the circustances of millions of households all in different circumstances and there are going to be occasions when one thinks hard, assesses the risks, and then decides to breach some of the rules.
I did this when DD was ill and we had her home a few days before the introduction of bubbles made it legal and I am doing it now, DS lives 200 miles away and has come down for the weekend to see his DF and support me and, as a result, is staying a couple of nights in the house.