I think, if I'm gathering things correctly, it's the effect on herd immunity threshold which now takes a massive increase due to the transmission difference.
If 67% more transmissible (as per PHE report) I'm reading that takes HIT up to 87%. If we have a vaccine which is 90% efficacy against symptoms, this requires 96% of the population to be fully vaccinated. As we have people who cannot take the vaccine for medical reasons and some who point blank refuse, this results in this figure actually requiring infections rather than vaccinations to reach that threshold.
The difference between this variant and the Kent variant is actually bigger than what the difference between the Kent variant was to wildtype, which doesn't spell good news for the rest of the globe and drives the probability of further yet more harmful mutations.