I flew into LHR at the beginning of September. I'd completed the locator form online. E-gate did not work with my passport (seems to happen about 50% of the time in T5 and I only pass through a couple of times per year) so I went to a "manual" gate. Was asked to show my passport and form on my smartphone. They scanned the QR code on the phone and waved me through. From what I could tell, everyone who had to go through manually was being asked about the locator form and was being checked to ensure that one had been correctly completed. Not sure if the e-gates link to check that you have completed a form.
At least this time the ANPR cameras on the car park worked both in and out. Usually I have issues with them reading the car plate (always clean and in the only legal UK font but it's an old- non-dating number) and numerous emails to complain and ask for their software to be checked/improved bring the typical "it's not our problem" response. It once (May 2019) took me half an hour to get out with 2 members of staff coming across to the car at the exit barrier, confirming on the intercom to their superiors that I did have the booking correct and confirmed on my smartphone but they still could not find a way to open the barrier...... doesn't inspire me with much confidence