Fog / Fire Cat
I don't know about Biggin, but Southend can upgrade their fire category to Cat 6 with a maximum of one hour's notice so, in effect, can always offer this for LCY diversions. Yesterday they were providing Cat 6 all day in anticipation of the dozen or so diversions they received.
I would have thought that Biggin might have a problem processing the numbers of pax involved, but other than that they should be a suitable alternate - certainly for LCY bizjet diversions.