The 7-across Trident 1E was an order for five aircraft for Channel Airways, a real bottom-feeder carrier of the time where Sqn Ldr Jack Jones, who ran it, packed as many seats as he could into their fleet (quite small but very varied).
Their Tridents were an extraordinarily complex type for an airline who had previously had nothing bigger than Viscounts or DC4s, and was compounded by their main base at Southend not even having a runway long enough to operate them from (I once used up most of it landing a Cessna 152 there, but that's a different story
), so their Tridents had to operate from Stansted. However they were also short of cash, as ever apparently, so three of the five were never delivered and just sat at hatfield, one was eventually sold by Hawker Siddeley to Ceylon and two more to BKS, Channel only taking delivery of two aircraft with this seating arrangement. They still had no real good Mediterranean charter contracts for them, one aircraft was finally placed with a West Berlin travel company operating from Tegel, while the other was grounded AOG at Stansted being steadily robbed for spares to keep the first going, Channel having run out of any credit with Hawker Siddeley by this time (they simultaneously did the same with BAC for some new One-Elevens they ordered). When Channel went under early in 1972 the Tridents joined BKS, later renamed Northeast, which was owned by BA and that's how they eventually all got onto the BA fleet. The 7-across seating had long gone. I bet the maintenance records raised some eyebrows at Heathrow.
You will notice the squashed seating is only in the forward half of the cabin, it was 6-across at the rear, as I understand HS could only make the evacuation limits work for this number. The smaller seats were (in theory) assigned to families with small children, but I bet that never worked out on the day.