The assessment of punitive fines for missing school is a new-ish government policy from what I understand, that has only gone into effect in the past year.
Here is a link with the government policies on school attendance and absence:
https://www.gov.uk/school-attendance-absence/overviewDifferent schools seem to interpet and apply the fines to greater or lesser degree, so your mileage may vary. I do know of a woman who took her son from school to go visit family in the US for a week or so during term time, and
both she
and her husband were charged a £60 fine
each, which is pretty harsh IMO. And that was when her son had no other school absences at all previously - except for when he had chicken pox. He'd previously received a perfect attendance award.
A lot of parents aren't very happy with the policy and some will argue it is nothing more than an unethical money making scheme for the schools. I've also heard of parents who are thinking of home schooling their children here - not because of conservative or religious reasons etc, but rather to not have to deal with schools (and Dept of Education) having more say over what their children may or may not do than the parents do.