AFAIK the only legal issue is that of liability, BT might charge you (or try to) if they in the future need to do work to correct changes you might have made, but afaik the actual legal reason (its not criminally illegal as such) you shouldn't touch it is due to liability issues i.e. if changes you made behind the faceplate resulted in mains electricity flowing into the system and electrocuted a BT engineer working downstream then you'd be liable and prosecuted, their end of the system is supposed to have protection against things like that (or atleast the liablity is on them if it doesn't) mess around with it and you open yourself to all sorts of liability issues that could potentially result in manslaughter charges, etc.
I understand they consider 'tampering' to be liable for prosecution and/or temination/disconnection from the BT network. Ie. bt have the right to protect their infrastructure from damage. I've searched online, but can't find the exact legislation.
