Bengerman
As said earlier a FULL EMERGENCY is just an ATC phrase that triggers a certain response from the emergency services. Theses I should add are universal UK ATC terms defined by both SRG in Mats Part 1 and also the CAA in the booklet 'Model Aerodrome Emergency Orders.
As for the term fully ready...well.......lets just say that certain of us have been caught out by aircraft calling for start when they are still loading pax/doors open etc etc. Hence we often confirm with the term fully ready - this is more important now we have the 15 minute rule whereby we are not allowed to let you start/pushback if your are 15 mins after your FPL EOBT.
Same for pushback really...have been caught loads of times by aircraft calling for push when they still have the doors open!! I am sure this is the exception rather than the rule but it can cause us problems at times. I know a certain Captain who actually uses the phrase himself!!
