Is it not more to do with the fact that the LH 767's are being dusked (flat beds etc) and that the new services to Shanghai etc result in more 777 flying. These facts are driving the requirement to fly some of the SH 767's on the TLV, for example. These will be flown by SH CC. The SH 767's flying some LH routes result in more work (again!) for the stretched Airbus fleet with talk of Lazarus (in the shape of the 737) having to return to LHR for some rotations.
As has been said before elsewhere, CC can only work 3 types of aircraft and at LHR we have A319/20/21, B757, B767, B777 and B747 - 5 types - making mixed flying less likely than at LGW with only the B737 and B777.
The B767 is unique in that it flies both SH and LH and as the LH CC are rumoured to be understaffed through the summer the plan has been developed to cover some of the shortfall from SH.
Still, never let facts get in the way of a good rumour!
[Edited to add:]
There may well be the occasional LH aircraft operating ad hoc SH routes - CDG recently springs to mind - they always have, BFS on Tristars for Xmas, LIS in Euro 2004 on B747, etc. They may, like the above be planned but they will be rare.