Without being a member of a union you are able to ask a representative to accompany you to any meetings with your employer ('right to accompaniment' Employee Relations Act 1999 Section 10 creates a statutory right for a worker to be accompanied by a fellow worker or trade union official of their choice during grievance and disciplinary procedures).
If your employer recognises a trade union it will say which one(s) in your contract. Most Union websites have a lot of information available for free; the larger ones, Unite, Unison, GMB are packed with information!
However, you may not be entitled to legal support or further advice without having been a member for a few months beforehand.
It is important that if you are a member you approach your union before seeking further legal advice as they may not be able to support legally if you have obtained advice elsewhere.