"politicians have to make up their own minds based on their individual conscience"(see BBC report Abortion 'is a conscience issue')
Apparently, Kelly was responding to Cardinal O'Brien's excellent sermon in which he said that Catholic pro-abortion MPs should consider whether they should receive Holy Communion.
As happens so often, the mantra "conscience" is utterly misused. If a person's conscience told them that they had to slice off their grandmother's arm, they would have to follow it. Nevertheless, it would be an erroneous conscience and the police, the Church, and anyone else of good will would be quite right to stop that person by force. The Church in particular has the duty to teach them the error of their ways and to instruct them so that their conscience might be better informed.
Kelly equates "conscience" with "deeply held conviction" which is not the same thing at all. A Catholic politician who thinks that their conscience (or "deeply held conviction") tells them to vote in favour of killing babies in some circumstances first needs to be convinced (as Newman pointed out) that it would be a mortal sin for them to follow the teaching of the Church. If they really think that, it is surely not a great leap of the imagination to realise that Holy Communion is out of the question.