well if you fail the roadside emissions test it is a whopping £60 fine... thats if you get pulled over for testing in the first place. and the chances of that most likely depend on how loud your exhaust is, and how smoky your car looks - i think the scooby boys and other turbo'd cars will raise far more suspicion (admittedly they have more to gain from a free flowing exhaust)
as for the insurance, id be surprised that they would think to check if an exhaust has a decat pipe or not, depending on how many other modifications the exhaust has.
in my opinion, the only reason i wouldnt run decats is the difficulty of getting it through its MOT, but if you've got that covered i personally wouldnt let the risk of a roadside emissions test or a insurance discrepancy stop me. My previous car ran decats and an illegal numberplate (correct font and spacing, but it was tiny lol) and wasnt ever stopped for either. might be worth searching to see if roadside emissions tests are common in your area, but as i say, there are quite a few other cars that would be more suspicious.