There are a number of different teacher unions which you can join. The main unions are outlined here:
For more information, visit NASUWT | The Teachers' Union
For more information, visit National Education Union | Home (neu.org.uk)
For more information, visit Home (naht.org.uk)
For more information, visit ASCL - Home
For more information, visit Community Trade union - We’re a modern trade union, and we’re here to help you. (community-tu.org)
For more information, visit Edapt - Edu-legal support for teachers and school staff
For more information, visit UCU - Home
For more information, visit UNISON - the public service union