Continuous video surveillance on 3-4 km of runway, from multiple angles, with night vision support, working 24/7? Might not be impossible, but surely not cheap.
I watch landing and takeoff videos frequently on YouTube from only one angle. The lenses are quite large and depth is very compressed but it’s sufficient to understand a lot of things relevant to an accident.
Daytime only would still be a huge improvement.
There's enough plane spotters around Heathrow 24/7 that this is actually probably covered.
It would take a heck of a long lens to have this on video from Heathrow.
Might need a couple of mirrors too...
Many airports do have cctv, ntsb’s channel often include those footage