Through the end of 2005, total North Sea oil production--as predicted by the Hubbert/Deffeyes method--is down 25% from its 1999 peak.   I would call that a collapse.   Given the estimate of remaining reserves, I don't see any alternative to continued sharp declines in production.
The UK part of that has droped from 2.693mbpd in January 2000 to 1.378mbpd in September 2005 a drop of 51% That must be a collapse by any standard.