Yes, it's determined by the location of the flight not the location of the aircraft's or airline's registration - Art. 3.1(a) and (b) of the Regulations:
http://news.bbc.co.uk/2/shared/bsp/h...egulations.pdf
Or for a more user-friendly interpretation specifically looking at the volcano disruption:
http://www.auc.org.uk/docs/306/AUC%20Volcano%20pack.pdf
Pohutu