You do not have to belong to a Teaching Union. SupplyBag.co.uk however, highly recommends that you are a member of one. If you do not have membership, then please look into Professional Indemnity Insurance, try HISCOX for a starting quote, and make sure you have legal expense cover.
Some Teachers Unions have different membership fees depending on how much you are expecting to work as a supply teacher and on part-time contracts. For example, the NUT for one, has one price for supply teachers (paid daily/hourly) and a pricing structure based on you working up to 30% of a week, 30% to 60%, or 60%+. I update my details once a term as contracts begin and end to make sure I'm not paying too much or too little.
Please find below links to the main Teaching Unions in the UK:
