They explained this - it was to do with a protein that enables the virus to get into cells. Explained here
Different SARS-CoV-2 strains haven’t yet had a major impact on the course of the pandemic, but they might in future.
www.nature.com
There isn’t enough information to prove anything, but science is about hypothesising first and then proving, changing the hypothesis as new information is presented. It might not be 70% more transmissible, it might be resistant to the vaccine, it might be more deadly....the current hypothesis will be based on existing knowledge of viruses and initial findings about this particular mutation.
They are guessing and advising on a worst case scenario. Politicians make the final call.