Alan (airchabum) from flight ops said that some of the LHR-MIA flights were planned on an LGW suite 744 during early 2005.
My guess is that it could be because they need extra Premium Economy availability during these periods, while at the same time Upper loads are quite light.
In previous years, I'd guess the usual thing would be to oversell W class and op-up the overspill to the J cabin. With the Suites, it seems that VS are trying to reduce the number of op-ups they have to do, by avoiding overbooking in W, hence a need for a bigger W cabin to match the predicted yield.
This will mean an inconsistency in the service with other LHR flights (micro-bar and no ifbt), and probably make it much more difficult to find Z fares or G rewards on these aircraft.
I recall that the MIA flights were producing A343s for a while earlier this year, presumably due to predicted light loads.
I have a MIA trip coming up in early March. I wonder if we're back to the regular LHR config aircraft by then?
Mike