I think the general formation for an SR type 4EPB was DMBSO, TSO, SO, DMBSO. The 55xx series was created being DMBSO, SO, SO, DMBSO. The idea was that they would be the first batch withdrawn which I think they were.
The 2EPBs were more of a problem as they had half a coach as compartments but again, I think they were prioritized on peak hour workings.