Wasn't there so unable to comment. 15,000ft doesn't sound correct, max ceiling for the Canberra is about 54,000ft, Tornado ADV's are about 60,000ft + and probably a little less for the GR versions.
15,000ft probably relates to the type of image the Canberra can obtain maybe using a strategic mapping camera with "film" using the visible part of the spectrum as opposed IR and side scan radar images.
As an aside, PR aircraft always seem to come from existing airframes, which are then adapted, can't think of a purpose built PR aircraft? I may be wrong, even aircraft like the Blackbird SR-71 was originally an interceptor. If memory serves me correct before the Canberra it would have been the Valliant’s, Victors and Vulcan’s from the V force that would fulfilled this role.
Perhaps Mark can help us out. I've always been fascinated with reconnaissance.