Taking your child out of school
In England, Wales and Northern Ireland you write to the school and that is usually it. In Scotland you need the council's consent. A child at an online school counts as home educated in law, so the home education rules apply.
England
The law says a child must get a suitable full-time education "either by regular attendance at school or otherwise". To take your child off the roll, tell the school in writing that they'll be educated otherwise than at school from a given day (regulation 9 of the 2024 registration rules). You don't have to give a reason. There are two exceptions today: a child the council placed at a special school needs the council's consent, and a child on a school attendance order needs the order lifted first. A child with an EHC plan at a mainstream school needs no consent.
The DfE's 2019 guidance says parents can use "online tuition" as part of home education, and the accreditation scheme's own guidance describes parents using online schools to meet their duty. Your council may ask how your child is being educated; a timetable and the school's details usually answer that.
This will change when the Children Not in School register starts: you'll have to register, and some children will need the council's consent to leave school.
Wales
The same written notice works (regulation 8 of the Welsh pupil registration rules), with the same special school exception. Wales has statutory guidance for councils and no register yet.
Scotland
A child who has been at a council school can't be withdrawn without the council's consent, which "shall not be unreasonably withheld". The 2025 guidance asks councils to decide within six weeks. You don't need consent if your child has never been to a council school, is leaving an independent school, or is between primary and secondary.
Northern Ireland
The duty is the same as England's (Article 45). nidirect says to tell the principal; no permission is needed, but tell the Education Authority if your child is leaving a special school.