Assuming all the 'paperwork' is now pre-filled electronic documentation, with the onus of steep fines if incorrectly/fraudulently filled out by passengers ,and knowing exactly who was on each flight between a few hours to a few days beforehand, and having more than enough staff to cover all the arrivals into LHR (because the passenger volumes are way down) and the main cadre of Border staff are furloughed, but ready to start at relatively short notice, how could there be so few staff ready to process the arrivals ?
Might it be a deliberate strategy to further dissuade arriving passengers attempting the journey now the UK is serious about controlling it's borders ? Self-financed quarantine hotels, self-financed covid tests x2 and now ominous waits after perhaps travelling for 12hrs plus. It just seems either incredibly cackhanded, or a planned obstruction to broadcast 'stay away' at this time.