Settlement calculation for term-time employee.

When employed term-time only but paid worked weeks plus accrued holiday entitlement in 12 equal payments, what calculation is used to work out the weekly rate of pay to then calculate a settlement?

Also, if the employee has been on long term sickness absence, are they entitled to the holiday pay accrued?

Example (not actual figures) Employed 25 hours per week at £10 per hour. 40 weeks worked and 5 weeks holiday entitlement given.
Assuming holiday is accrued at 5 / 40 = 0.125 of a week per week.

Annual salary = 25 * £10 = £250
£250 * 45 = £11250
Paid at £937.50 per month over 12 months

Example of proposed settlement
12 weeks notice pay
10 weeks settlement (equivalent to redundancy entitlement)

Employee has been absent on sick leave for 20 weeks, 15 of which we’re term-time but all covered by a sick note.

I assume that the weekly rate would be £250 therefore the proposed settlement total = 22 * £250 = £5500
Also assume the employee would be entitled to payment for holidays accrued during term-time while on sick leave of (15 * 0.125) * £250 = £468.75

The employer claims the weekly rate for calculation is the annual salary divided by 52 (eg. £216.35) And that holiday pay forms part of the salary so no holidays are accrued and no payment is due.

Can anyone offer advice please?