It's going to take many months (over a year?) to fully distribute, and the first few months of vaccinations are going to deliver the most important phase IV data points. Waiting just a little bit longer isn't entirely unreasonable. Why take chances if the Brits and the Americans are willing to do the final tests?

The main reason why vaccines are so safe (and why the anti-vax movement is so silly) is that they've been tested on hundreds of millions of people. But until you vaccinate those millions you cannot be absolutely sure about the safety.