No, it isn’t actually ?
Even if flights back from India had been stopped weeks before, the variant was already here, brought back and spread especially in the Asian community.As many Asian people do not want ( or rather fear) having the vaccine, then they were an instant target for Covid, probably spread by younger people from household to household.Then it was quickly in the wider community from schools and Colleges perhaps.
The vaccination programme was going as fast as possible, so nothing wrong there, but impossible to contain until just about everybody has now had two jabs.Variants in other places have proved to be the same.
I also think that flights from India and elsewhere should have been stopped earlier, but that wouldn’t have helped much and it would have been a very unpopular decision to leave UK residents stuck in India.