All SU flights from LED are operated by Pulkovo (at least all the ones to w.europe), wether they have a "Z8" flight number or not.
Pulkovo fly the Tu-154 to LHR in the summer (every Saturday) and change it to a Tu-134 in winter. The schedules for LGW are constantly changing, but usually 3-4 times per week.
Very rarely, you might see a Z8 coloured Il-86 at LHR. Maybe that's what you saw, but I doubt it. An error with their video editing? showing the inside of the wrong plane?
The one thing you can say about Pulkovo, is that all of their aircraft look great. Clean, bright, white paint and no jet blast stain on the tail. Always come out well on my photos. A real treat to fly in too.