A consequence of falsely claiming to be a solicitor is that they may be prosecuted. There is a set of "reserved legal activities" under the Legal Services Act of 2007 which require a person to be authorized to engage in such an activity, but rendering stupid opinions about how the law works isn't one. Let us suppose that there is some common understanding in the UK of what it means to be a "lawyer" such as "has studied a bit of law", and person A giving the advice is actually a Russian plumber who have absolutely no idea about UK law. In which case B might try pursuing a legal remedy on the grounds that he was tricked into working for free by a person who fraudulently claimed to have material knowledge which he did not have.
Given general understandings of fraud, it could seem that the company attempted to use deception to gain something of value (work) or to deprive the person of a right (liberty). The Fraud Act of 2006 under provision 11 Obtaining services dishonestly indicates that one can spend up to a year in the clink for doing this.
This scenario is bizarre enough that I wonder whether we're missing some details. For example, perhaps B is already an employee of the company, he has a contract, and it stipulates that they can require him to in some form "work for free" as long as they give him notice. (Obviously this can't override regulatory law, but a company can make a non-hourly salaried employee do all sorts of stuff above and beyond the regular 9-to-5). Handing him this paper may constitute "giving notice": the signature would be acknowledgement that he had gotten notice.
