sticky sherbert said:
She says that in her local authority they never paid any term time only employees any holiday and they used the national terms and conditions so it is probably the case with your wife's employers also.
This is from the direct gov site but note it starts with the line "will depend on your contract of employment"

which sort of leaves it open
Term-time workers
If you only work term-time your holiday arrangements will depend on your contract of employment. If you work a reduced number of weeks during the year, you accrue a pro-rata entitlement to paid leave. You need to calculate how many hours a week you work on average over the whole year, then multiply this by the holiday allowance.
For example: your contract is for 40 hours a week for 40 weeks of the year, 40 x 40
Step one: multiply your weekly contracted hours by the number of weeks you work:
40 hours x 40 weeks = 1,600 working hours for the year
Step two: calculate the average hours you work each week by dividing your hours for the year by 46.4 (which is 52 weeks in the year minus the 5.6 weeks you would be on holiday and so not working to accrue annual leave):
1,600 hours divided by 46.4 weeks = 34.48 average hours a week
Step three: multiply your average working week by the holiday allowance:
34.48 hours x 5.6 weeks = 193.09 working hours' holiday allowance a year
Step four: if you want to convert this into holiday days, then divide again by the number of hours per day that you work.
Taking your leave
If you work your full 1,600 hours and take your holiday at other times (eg during school holidays) you accrue holiday on the whole 1,600 hours which equals 193.09 hours.
If your contract of employment requires you to take your holiday during term-time, you accrue holiday on the weeks (or hours) you actually work.