Multi-element flypasts have the successive formations stacked up and down by a couple of hundred feet to minimise wake turbulence issues and to provide a margin of safety in case of timing errors or loss of visual contact. When the cloud is as low as it was today, the number of available flypast levels between the minimum safe (dictated by the City skyscrapers, which are close to the flypast line) and the maximum legal (dictated by whatever margin from the cloudbase has been agreed between the various authorities) is reduced to the extent that aircraft have to be removed from the flypast in order to avoid stretching it out into an over-long snoozefest with IFR separation between elements sharing a single level. It's nothing to do with the all-weather capability of the individual aircraft involved; any of them could have flown through at the level used by RAFAT.