Not wishing to comment on the specifics on this case but to get access to coaches to drive the pax to LHR is not as easy as it sounds.
It is possible that Air India did not have a handling presence in Gatwick and that makes arranging any thing extremely difficult. Such as access to a gate or stairs and the like.
However, having said that, 8 hours on the aeroplane does sound excessive and I am sure questions will be asked.