They used the middle lanes on the boat & seemed to have enough
headroom, so it just night be a regular occurance, but we were
wondering why said busses would use the ferry rather than a bridge or
tunnel?

Only speculating but if you want to get a double decker across the
Thames for service in E London the ferry is a good deal easier than
driving to Tower Bridge or Dartford crossing? (Blackwall is OK
Southbound IIRC for a stock DD but def. not Northbound in the old bore.)

IIRC much more common in days of yore (before the Dartford tunnel) when
buses on daytrips from E London headed Sarf.

--
Robin
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