I had a little Google.
from the NASUWT site: https://www.nasuwt.org.uk/advice/conditions-of-service/teachers-working-hours/directed-time-england.html
1265 hours a year are allocated by the school. There is indeed an “any additional reasonable hours as necessary “ clause that’s in most salaried contracts in most sectors.
Interestingly, there is a specific paragraph stating teachers aren’t supposed to be asked to work at lunchtime - it’s not part of their hours. Which means that if it’s been happening in these schools then the department has been getting away with this for years, and then blaming the teachers when they say no.