I too spoke directly to VOSA about CAT/DPF removal.
Basically there is no testing criteria for DPF presence. Standard smoke test for particulate pollution is the requirement.
From a ECO sensitive and insurance point of view then if you have a DPF and it is working fine, etc. etc. then I would leave it in place. Yes the MPG is reduced but removing a DPF filter and updating the ECU software is a vehicle modification as far as hehicle insurance is concerned. You IMHO should declare this to your insurance company to be legally fully insured. I don't want you anywhere near me if you have done the mods and not notified and got insurance from your insurance company!
Sceptical? Trust me. I went into great detail when I was doing Motor Sport, sprinting, etc. in our road cars. Simple required RAC MSA stuff like yellow tape on the -ve battery lead, ignition on/off direction labels, etc. technically count as a reporatable insurance modification IF they are present on your vehicle on a public road.
I used to see competitors taping up on track arrival and removing prior to departure.
Just please be carefull. Look beyond the mods to the possible legal implications. You don't want to be paying damages for the rest of your life should you be the cause of an accident and then find yourself uninsured due to a stupid bit of sticky tape.