This letter is for those living within the England and is legally correct as of the date published. Things are different in other parts of the UK and will require more than the letter below. That said it is not impossible.
After you have sent them this letter (an in paper hard copy that has to be signed for is best) the school can send you a receipt but they can not legally harness, call for meetings (you don't have to go) or refuse to de-register. If they do, they are breaking the law.
At home education house
City of Rock and Roll.
Name of school.
Address of school.
Re Child/children's names d.o.b date of birth/s
Dear Headteacher's name,
I am writing to inform you that my child/ren are receiving an education at home, otherwise than at school, accordance with Section 7 of the 1996 Education Act.
As of this date, (date) remove their names from your school register; in accordance with the Education (Pupil Registration) Regulation 2016 Section 8 (1) (d) for mainstream schools.
Please confirm in writing you have removed my child/ren from the schools registers as of the date above stated.
(your name in print)