Unofficial income is income raised by the PTA or any such organisation from extracurricular activities or income given by an organisation or individual to the school as a donation.
Official income is income:
Annex D to the VAT Awareness Manual (pdf) (accessible to headteachers and bursars with a username and password) states that official income includes all income except donations, commission on photographs, school sales of sweet and soft drinks, and school trips.
See
section F of the schools' finance handbook
(word) for more information on income.