The official figures for the engines at mach2 are:
At take off and during subsonic flight, 82% of the thrust is developed by the engine alone with 6% from the nozzles and 21% from the intakes
During the Supersonic cruse only 8% of the power is derived by the engine with the other 29% being from Nozzles and an impressive 63% from the intakes.
As for viewing don't go anywhere near T1,2,3 the viewing areas are closed.
have a look at this page
http://www.concordesst.com/retire/viewconcorde_r.html
Remember Concorde currnetly only uses the north runway for depts, and for the next 5 saturdays there is no evening JFK dept as the summer and final BGI season is underway with a 09:30 dept (from t4)