Johnny37. That depends what it is.He isn't allowed to tell the police what the client tells him, nor anything else which falls within privilege.Don't worry too much about that. There are exceptions to privilege: not everything which would fall within the broad definition of privilege is , in law, privileged. For example,any communication which is made for the purpose of obtaining advice on the commission of a future crime , or which is itself part of a crime isn't protected.(In one case a solicitor was sending menacing, abusive and obscene telephone calls on behalf of his client. Not a good idea: that was itself an offence under the Telecommunications Act 1984, quite apart from what the Law Society, his profesional body, would say!) Likewise documents obtained in contravention of domestic or foreign law canot be privileged.
Concealing evidence? Starting with speaking:even your supposed obligation to speak has limits. There is no such broad rule.Keeping silent is not an offence in itself, whatever you've seen or heard.You don't have to volunteer yourself as a witness.There are a few crimes of silence but they are special cases. One is 'concealment of evidence': X has committed an offence and Y knows that. Y has information which might be of material assistance in securing the prosecution or conviction of X. Y then accepts or agrees to accept payment for not disclosing that information e.g (X 'buys his silence'). Y is guilty of that offence.
As to physical evidence, there's no general obligation to produce or reveal the whereabouts of objects or papers which might be of use in a criminal investigation.Trouble is that a positive act, not doing nothing, which tends to pervert the course of justice is an offence and, of course, police may (subject to various rules) conduct searches and seize property and you are not allowed to obstruct them in so doing.