It comes from a recommendation from the independent electoral commission:
“Voters should be required to show ID at polling stations in Great Britain to tighten up the security of the voting process, the elections watchdog has said.
The Electoral Commission will now develop detailed proposals for how the scheme should work.
Evidence collected by the Commission in its review of electoral fraud revealed that fraud is not widespread in the UK but, despite this, a significant proportion of the public remain concerned that it is taking place.
This has the potential to undermine confidence in the electoral system if not addressed. The Commission considered views on a number of options to tighten the electoral system against fraud and is today publishing its final report.”
Of course its already a thing in NI, and most of Europe.