Knee replacement - Recovery - NHS Choices
'If you're generally fit and well, the surgeon may suggest an enhanced recovery programme where you start walking on the day of the operation and are discharged within one to three days.'
Coupled with the high probability that the NHS bed on that particular day was needed for someone with a life threatening issue as opposed to recuperation that could be adequately carried out at home.....in and out on the same day appears to be a distinct possibility in certain circumstances