truth is they dont know
https://www.sciencemag.org/news/2020/12/mutant-coronavirus-united-kingdom-sets-alarms-its-importance-remains-unclear
"One, called N501Y, has previously been shown to increase how tightly the protein binds to the angiotensin-converting enzyme 2 receptor, its entry point into human cells."
tightly bound also means bound for longer
It has long been recognised since 1665 and probably 1348 that as a epidemic goes on, the cases get worse
["passage thro the host increases virulence"]
and this has predictably happened here
so they have the 17 changes that happened at once in the virus,
and they know it is more aggressive
but still nowhere near: " you see that mutation at B15? well that twoingies the blong-blong, the dongles droop and the femta falls and turns it into a killer virus"
even saying - and the R number goes up by 0.5
all that means is that more peoplle get it from one person, around 50% more than before