I don't necessarily think that GB have been sandbagging. What may have been going on is that they've intentionally structured their seasons to work around the WC season.
Training is not a steady progression, but a series of "mini-peaks" that lead to a final Peak Event. It could be that they have intentionally scheduled their training programs so that the valleys in between the peaks are coinciding with major events. They are still progressing throughout the duration on the program, but essentially are doing just enough to qualify for the major events, and keeping their A Game to themselves to keep everyone else in the dark.