I do BA, SU & UN on the route.

Pricing is easy to understand.

BA are the only airline that has a Saturday Night Stay restriction. Factoring this in means you can usually get Y on any at 250-290.

UN & SU sell one way at half return, BA like most western airlines does not.

At school holiday times all of the airlines go crazy expensive LHR-DME/SVO/VKO at the start of the holiday period and the other way at the end. I needed to return on the Saturday the holidays started and there were only a few seats left on any of the airlines and UN at £1800 was the cheapest. Used Avios for a y Ticket and got an OPUP on the overnight, so very happy. This happens every year as the kids in school here go home.

As to loads. There is a pattern to it. Business in on Sunday Night/Monday Morning flights and out Thursday/Friday flights. First and J rammed on Sunday's and Monday's LHR-DME and the same DME-LHR on Fridays. lighter loads on Tuesday-Thursday flights. Y varies from about 50% to full the whole week and is mostly Russian. Down a fair bit due to currency fluctuations. The night flight always seems full in Y.

If like me you live and work in both cities it's great for redemptions. Fly to Moscow on a Tuesday or Friday and a redemption is usually available, fly the other way on a Saturday or Thursday and it's the same.

as to why a wide body When on some days it seems J and F can almost empty. Firstly, that seems from my experience to be a one way thing, empty in and full back and secondly I believe that Frieght is a key component of what BA carries on this route.

Service wise. BA and UN are the only operators of F on the route and both seem to run an average of half full. BA have the better product with UN's F being akin to CW.

For J. BA hard product is way better than the competition, CW seats compared to J which is akin to WTP with more leg room on the others, food wise, UN and SU selection is better, though I prefer the drinks on BA.

Economy Plus /WTP BA are way way better as on UN their Y+ is first three rows plus exit and leg room only delta. SU call it comfort and it's the front bulkhead row and two exit rows, again just legroom

In Y, UN and SU have the better product, more legroom, better food choice, cooked meals on all flights as opposed to BA's cold cheese bun thing.

Then there is EasyJet, too Orange, no Recline, goes to some airport near Brighton and that terrible place on the wrong side of the Pennines. Also their pricing is usually more expensive than the other three unless booked ages in advance