Have a look at how many times the word "reasonable" is used in employment law. Reasonable is never defined.
Back to the topic at at, I'm of the opinion that:-
A: Parents knew vacations would be more expensive in school holidays BEFORE they had kids. It is not a surprise!
B: If the holiday you're looking at is too expensive at the time you want, choose a different holiday. Instead of going to Spain, go camping for a week.
c: If your work holidays don't align with your children's holidays, then tough ****. Get another job or do day trips at weekends rather than week long holidays. Explore what Britain has to offer instead of a foreign holiday. Visit museums. Go for a picnic. Go for a bike ride. Have a staycation!
IMHO, there is no reasonable justification to take a child out of school for a vacation*
Disclaimer:- I have three kids - I've never taken them out of school nor will I ever do so.