The actual wording is "a place to which the public have access", which I, along with many other pragmatic types, interpret as "the front door is open". If we don't use this ploy then invariably the 'patient' is arrested to prevent a Breach of the Peace, taken outside, de-arrested and then section 136'ed. I've never bothered waiting for the crisis team to come out, and I've never heard of anyone else doing it that way either. Neither of these two options are, on reflection, probably not exactly what Parliament / common law intended, but they are reasonable and proportionate and get the job done. (These views, of course, do not represent official Met policy...)