You stated that consciousness should be the criteria. To the best of our knowledge that begins to emerge somewhere after 24 weeks.
The current limit is set at as it is because it allows enough time to perform the 20 week fetal abnormality scan with enough time for any followup investigations, decision making and actually carrying out an abortion.
We're not talking very many cases either. In 2014, there were just 1,193 abortions carried out at or after 22 weeks. Of those, 456 were because of severe abnormalities. The rest are mostly because of medical risk to the mother.