Scientists here have known about the new strain since September. When it was first identified, its behaviour did not seem to be significantly different, but they’ve been keeping watch on it.
In areas where the new variant exists, infection rates have increased rapidly. In the same period, the proportion of infections due to the new variant have increased exponentially, from 0% back in September, to 60% now. The new variant is out-competing the original virus. In affected areas, the rapid rise in infections corresponds very closely with the number of new variant cases. The obvious conclusion is that the new variant is more transmissible / infectious than the original virus.
Of course, as you say, there could still be some statistical confounding factor that has not been accounted for - but that’s looking increasingly unlikely. Scientists have held off making an announcement until they were reasonably sure, or the risk of not acting became too great.
To some extent, the scientist’s hands have been forced by government plans to relax covid restrictions over the Christmas period. To allow people to mix freely for a week would have invited disaster.
It has to be said, the international reaction of port closures and air travel bans looks more like a knee-jerk political response than a reasoned, scientific one. News of the new variant has been freely circulated amongst the medical and scientific communities for months. Anybody who has visited London between September and now may have been exposed to it. If the new variant is going to spread around the world, there is a high probability that it has already done so.