Another thought about the London end of it JMO was sparked a post earlier in this thread
We've been overlooking the Tube.
If he needed to go to Charing Cross to check in weapon at armoury, it's only a few stops on the Northern Line to Clapham South station. Met police travel free on the Tube. Even if leaving shift from Westminster, it's short Tube ride.
It's quite common for those working in central London who want to avoid congestion charge and sky high parking fees to rent a driveway, and then do the last bit on public transport. If he had done that, it's a short walk from the Tube to Poynders Court, and you'd easily see someone crossing over from the Common and continuing along A205. He could have followed her from there? Seen her put her phone away and bundled her off the pavement into the darkness round the flats and in the direction of his car?
Also there are at least two takeaway places (one clearly open for socially distanced customers) between the Tube and the junction with south side and A205 which he would walk past frequently if that was his route. Obviously if The Sun report of it being the MaccyD are correct, they are irrelevant. But other accounts talk of more than one offence, or describe it as being in Clapham (which these are, but Brixton Hill isn't)
MOO