Finicky this one. You are correct that it's not actually transit if you return to the point of origin. If you got to DME you would be able to reverse back without issue, even though it's not within their definition (more a factor of the layout of DME). However BA have programmed a Visa Check stage to Russia (and various other countries) into Ready To Fly, so you cannot avoid trying to explain this lack of visa in T5 and I don't think the agent would let you do this with 100% certainty. There are reports here of people being able to do it, however. Russia allows genuine transit however, without a visa, so if you were returning (e.g.) via Warsaw then that would be fine.

[China is the place where you definitely can't do this, they have a visa free short stay for transit passengers at some main cities, but when they mean transit, not return to the point of origin].

In any case, Moscow is a fascinating place to go to, it's quite a long way to go, so I'd make a weekend of it.