Dont know for sure but from my last couple of flights they just semed to be checking the scanned data matched the printed page.
I remeber reading they recorded people entering but only kept the data for so long. As there was a big ho ha in the papers regarding it a few months back if i recall correctly.
To be honest recording the 10's of millions of flights which enter and depart from the UK would probably mean visas would reach even greater amounts.