The interesting thing is the Operation section sounds a lot like they're using the same principal behind TEMPEST [0] and van Eck phreaking where leaked emanations from electronic devices can be used to spy on the computation being done and data being displayed. Flat panel displays can be spyed on using this method [1] so it's not completely out of bound of reality that these vans could work as detectors.
It's very likely that those vans were used in the 1970s to capture the RF leakage from the local oscillator and other driving circuitry of the CRT TVs. But while it's still technically possible to detect RF leakage from modern LCD TVs, I'm not sure BBC has the capabilities to deploy those and achieve reliable detection. Many people would say these vans became hoaxes at some point in history.
[0] https://en.wikipedia.org/wiki/Tempest_(codename)
[1] https://www.cl.cam.ac.uk/~mgk25/pet2004-fpd.pdf