I came upon this problem around 20 years ago when, as assistant general secretary of the Professional Association of Teachers I was charged with creating the administrative structure of the Professional Association of Nursery Nurses.
Once PANN had been registered as a trade union and had obtained its certificate of independence, I set about obtaining tax relief on the subscription. I soon discovered that being a trade union was irrelevant, the significant criteria were concerned with an organisation being a "professional body" concerned with all the professional, as distinct from the employment, issues of a job.
I was able to demonstrate that PANN, like PAT, was concerned with professional issues as a "professional body".
I can only conclude that there are some teacher trade unions that need to revisit the Inland Revenue and to demonstrate that they too are concerned with all the professional issues relating to the job of a teacher.
John R Andrews General secretary, PAT (1992-1997) 1 Chaseley Gardens Burntwood Staffordshire