as LoBelly and Treeve say, sidepipes themselves are not inherently illegal.
People say they are illegal because they USED TO BE. There was an exhaust emissions ADR which prevented any left side-exit exhaust on category N vehicles, this ADR was removed quite a few years ago.
You do however need to meet noise requirements, and may need to provide heat shielding.
I say may on the heat shielding because the ADR states something along the lines of "general safety requirements" and this aspect of heat shielding sidepipes, what material, how far it extends, etc etc, is at the discretion of the engineer. My engineer has told me that I will need heat shields.
Noise is noise, you are unlikely to get them to pass the tests. I'm pretty sure mine are going to be too loud, but jeez they sound awesome - lots of snap crackle pop when backing off
winner winner.