pulled over on suspicion of not having a catalytic converter/possible fine for it being on a flange

i got pulled over last night while taking my (road legal, with mot, tax and full insurance ect) track car on a short test drive as id just rebuilt the bottom end of the motor, i was going under the speed limit when an unmarked car behind me put on their lights and signaled for me to pull over. the officer who came and met me accused me of not having a cat on my car as he "could smell it" and the fact the car is loud, i explained that yea it is loud but it does have a cat and i had just passed an mot and still even had the pass paperwork on my passenger seat, which i showed him but he said i likely just removed it after the test and demanded i show him that i do have a cat which involved me crawling on the floor in all the road grit to take a picture of the cat which was honestly not a great feeling to have to be doing.

when i showed him the picture he accepted that i do in fact have a catalytic converter, but he also mentioned that he noticed that the exhaust has a flange in front of the cat (the end of the exhaust manifold) and one 30cm after the cat which he said was proof that i had modified the emissions system and i would be getting a fine for such. i explained that the section being on a flange lets me swap out the cat when im on track as not to damage it and allows me to replace just the catalytic converter once it falls below the useful threshold rather than having to cut and weld a new one every few years but he insisted that because i COULD remove the cat it was against the law and would be fined. he left pretty soon after saying that but didnt take any of my details or such and didnt hand me anything, i dont know if its done electronically now or if that means im not getting a fine?

is being made crawl under my car in the road grit something they are allowed to do?
am i getting a fine?
is the fact that i could remove the catalytic converter enough to get a fine?
if i do get a fine should/how do i fight it?