I get that we should generally redistribute things from those who have most, to those who need most.
I agree. In this case, the UK is one of a few countries who needs the most - it's one of the countries with the poorest record of deaths, infections and the springing up of new variants which take over because there's piss poor control over the virus....
If we say divert our vaccine, that we were going to give to say 40-50 year olds (exactly the age group that will have secondary school children) to poorer countries, to people with a much much lower chance of infection as rates of covid 100x lower than here. Then open schools with no additional safety measures until the rates in secondary pupils are where they were in December, then we have the virus ripping through the 40-50 year old parents. Some die, some kids bereaved, but more importantly from a global pandemic viewpoint, a lot of chance for new variants to get a foothold.
In the longer term, it might actually be better for the populations of the poorer countries to wait for the vaccine and there be no opportunity in the UK for a vaccine resisting variant developing.
For countries like Brazil, it's a different story as they've been affected badly and (I assume) do not have the healthcare capacity of the UK. But just to say give the vaccine to poor countries where covid rates are low is actually not doing those countries a favour. The best thing we could do is to aim for zero covid globally, which if vaccination goes well in those countries with highest rates of infection, might be possible (maybe I'm dreaming, but I think we have a window for this)