It's not about viability - this is a red herring. Medical science advances all the time, so the point of viability just gets shorter and shorter, hence giving the appearance that the time limit for abortions should move to keep in step with medical advances.
But some serious developmental problems can only be detected when the foetus is fully developed, and all the medical science in the world can't change when that occurs. Only at the 20 week anatomy scan can it be confirmed that a foetus has developed correctly. This confirmation cannot be obtained via blood test.
I know two women who had late term abortions, both because of serious issues discovered at the 20 week anatomy scan (which, despite the name, can take place at any time between 20 and 22 weeks pregnant). Neither foetus - both very much planned and wanted) would have survived birth and breathed independently.
One woman was in the UK - she was given a follow up more detailed scan 48 hours later to confirm, and was then admitted for a termination.
The other was in the US, in Texas, which had already outlawed late abortions except where there was a risk to the mother's health. To add insult, her health insurance was a Texan firm and didn't cover an out of state termination, so she had to find a clinic in another state, book in (having also to go through the verification scan process), then book flights and a hotel, and book a week off work (unpaid), all out of pocket - the cost was around $8k. Even her doctor apologised for the barbaric law which would have required her to continue the pregnancy until she went into labour at full term, even though the foetus was only surviving because it was in utero.
No change is needed to the existing position in the UK. No debate.