The reason smacking is not supposed not to harm the child is why it's still legal. Even so, the smack mustn't leave a mark or the parents will be prosecuted.
Even though are some restrictions, abortion is legal up until birth and the circumstances can be fairly liberally interpreted. You might remember the case of the foetus that was aborted because it had a cleft palate and hair lip.
Abortion laws in the UK don't pertain to any rights of the foetus, because it doesn't have any.
The restrictions are for so called ethical reasons based on the fact that it's possible a baby born at 24 weeks gestation might survive, but not that therefore a right to life begins at 24 weeks.
I can see you don't believe me when I say foetuses don't have rights under European law:
http://news.bbc.co.uk/1/hi/uk/3878135.stm
and here:
http://news.bbc.co.uk/1/hi/health/3876311.stm
Nothing has changed since 2004.