Its not defined as illegal.
It is illegal to impede the safe navigation of another vessel, which in my mind means u cant anchor in the channel.
If i find a boat in the channel, and i have to pass closer than 30 metres to it, i wont slow down to six knots as im supposed to, for that reason.
u will find many different interpretations of the rules, and the truth is, none of them are wrong, because they are left wide open to interpretation.
Regards
Darren