The intumescent hoods in screwfix are not for draught prevention, they're to stop the spread of fire.
The gap between the joists should really be insulated, and there is no way you would be able to retrofit those hoods properly anyway, you could get tighter fitting downlighters.
I would suspect the downlights are not the cause of the coldness, the air coming in through them should be minimal. All my upstairs is downlighters and my loftspace is so well insulated that the loft is freezing in winter but I feel no draught through the downlighters. and they're uncovered above.
I'd look elsewhere for the chill, I think the downlights are a red herring.