Simple answer - WEATHER!!!!
The fast jet flypast normally forms up at Southwold and transits via Chelmsford to London. BBMF and the Egg beaters join up on the route after holding at Cambridge and doing a track around Stansted's approach area.
http://www.raf.mod.uk/rafcms/mediafi...665B79193C.pdf
The routes take the formations between Stansted and London City and puts the formation on an almost direct line on the Mall. There is no way that they can go south as that is the approach for London City and Heathrow, while access from the north is blocked by Stansted and Luton's approach patterns.
I checked the weather reports for the route at the time that the BBC said the majority of the flypast was cancelled. Most of the flight path over Essex, visibility was poor!!!! The Red's of course do formation flying day in / day out in the formation that they flew so they were happy to go at minimums, while everybody else were to be in mixed formations where they are not so used to formatting on the lead aircraft. BBMF, are of course are very much VMC only.