Just been thru all of this with my car and the engineer.
Basically....
Pod Filters in their own right are not illegal
The fact that they increase induction noise by a large amount on most cars is what makes them illegal if the car is subjected to the ADR "drive by" test of noise.
What I have done is mounted a pod filter INSIDE a factory airbox that had to be modified to clear the new (non factory) intercooler piping. The original panel filer could no longer be used, so a pod filter was the only other option.
Engineer looked at it last week and said great job, and passed it no problems.