I'm happy for Menzel that she's pursuing this. It definitely will allow her to "own" the role. I wonder what the economic motive would be. It's already a hot sell in London and since Menzel is a relative unknown there, her return to the role does not seem like it would boost the box office too much. Perhaps it will give the show more credential. Tourists feel safe that they are already seeing someone experienced in the role.
As for the prospects of an Original London Cast Recording, it's entirely possible. They could record it with Menzel, depending on who is in the other roles or they could wait until she leaves the show. As Michael Bennett has illustrated though, it's very uncommon now for there to be both a Broadway and a London cast recording. Producers can already sell the OBC in the theatre and at music stores. By making only one cast recording available, if people want the score, they will have to buy that one.
"I'm just a musical prostitute, my dear." ~ Freddie Mercury