A 50 pounder is a
consultant or advisor that gives unsolicited and useless advice, with the ulterior motive of getting paid for his apparently "quality" services. He is a salesperson, good with words, that can trick a naive person.
The 50 pounder is actually skint and has very little money in their bank account (about £50). When
confronted, the 50 pounder gets
aggressive due to his/her fear of being exposed.
To be avoided via good bullshit detectors and due diligence.