I'll keep it simple as well.
If a plane arrives at LHR BUT a passenger on that flight does not do a PCR test for 48 hours, but subsequently tests positive, there is a reasonable chance it "may" have been passed to other passengers on that flight some of whom may be in transit and exported it across borders before it is traced.
Yes they can be tracked down but isn't 48hours to capture a record a wee bit too late at a major hub like LHR ?