The short answer is no the next flight will not be held for you.
However with 2-3 hours between flights you should be ok barring any major problems.
You & your baggae should be checked all the way through.
A word of warning though. IB do not have individual inflight entertainment systems so bring an extra book!
For a really nice meal in Buenos Aires try 'La Bistecca' in the Puerto Madero (sp) area.
One other point, are you on through tickets or have you bought seperate ones?? By that I mean LBA-LHR-MAD-SCL or have you bought LBA-LHR-LBA and then a ticket on IB??
If you got through tickets then its the inbound carriers responsibilty to rebook you if you ae delayed and miss your connection.