Watch the programme. They had to trawl through 1100 hours of footage. They searched the flights in from Moscow for a week before the attack. They identified them from that then matched that when they discovered them on footage at all the places getting there and back from Salisbury.
However, people who want to believe conspiracies reject everything that contradicts that belief. So, I will bow out and let you carry on.
What conspiracy theory am I pushing ? I believe a russian spy (one of ours) told MI6 that agents were coming over, a surveillance team followed then round saw them approach the house, they presumably didnt realise what they were actually doing with the novochock, and thinking it was a scouting mission, let them fly back home. It was only when the two became ill was the true reason for the visit to the house realised.