If an employee does not return from adoption leave, does the employer have to pay in lieu of the annual leave accrued during the leave?

Yes. Employees on adoption leave are entitled to all their contractual rights (except remuneration), so annual leave continues to accrue during the adoption leave period. If the employee resigns at the end of the adoption leave period, the employer must pay them for the accrued untaken annual leave. If the employee resigns with the termination date effective before the end of their adoption leave they will be entitled to payment only for leave accrued to the date of termination.