An ambitious title, you may think, but one that is totally justified.

And the proof is...Time Crash.

In Time Crash, two versions of the TARDIS merge, so that the 5th & 10th doctors meet. BUT the 5th doctor is alone! (NB: if he had any companions with him, they would have appeared in the merged TARDIS like the 5th doctor himself did.)

Now, the ONLY chronicled period in which the 5th Doctor travels alone is whilst returning to Frontios after dropping off the Gravis on Kolkokron. AND since the BF audio Excelis Decays is set during that period, since Time Crash is canon, they both must be!

Therefore, if Excelis Decays sets the precedent, then ALL Big Finish Media can now be seen as canon! Q.E.D.!