Unfortunately as the law is so vague, and various cops are clueless as to the law...there is no easy answer

Any deviation from the manufacturers standard or modification to make the exhaust louder can be deemed illegal. Although it usually takes a wanker of a cop to enforce this.

Of course on the other hand, you get ballbags in their Honda's with ear splitting sounds from their cars who absolutely deserve all the grief they can get due to the exhausts...or lack of they do to their cars.

If you've a bit of sense, fit as many and as big a silencer as you can and keep the noise down.
Silencers are available for relatively low cost these days and in all shapes and sizes.