Documents containing material that attracts legal professional privilege cannot be seized – no warrant can authorise that. In R (Tchenguiz & anor) v SFO and Ors  1 WLR 1634 the High Court said (para 264) that it was clear that; “from R v HM Customs and Excise, Ex p Popely  STC 1016 and R v Middlesex Guildhall Crown Court, Ex p Tamosius & Partners  1 WLR 453 that the proper procedure is that an independent lawyer should be present to assess claims made for legal professional privilege, without prejudice to the right of the person being searched to go to the court.”
Often, where there is an obvious danger that documents attracting LPP might be inadvertently seized, the warrant will allow for the agency to bring with them an independent lawyer to review material in situ. Your own lawyer present at the scene can demand that documents are assessed for LPP – or that they are bagged separately for later independent investigation.
Special Procedure Material (SPM) also cannot be seized under a search warrant. SPM is defined at s14 PACE. It is a sweeping up category of material meriting special protection but falling outside of LPP and Excluded Material (journalistic material). This includes material in possession of a person who acquired or created it in the course of any trade, business, profession or other occupation, or for the purpose of any paid or unpaid office (s14(2)) and is held by that person subject to an express or implied undertaking to hold it in confidence. That sounds far reaching but in effect it applies to confidential information such as medical or welfare records of individuals or very sensitive business documents such as confidential contract negations or perhaps staff disciplinary records. Where SPM might be seized then the warrant has to be authorised by a Judge of the Crown Court, not a Magistrate.