You are quite right that after chemotherapy treatment, if there has been any hair loss, then the hair always grows back over the next few months.
When radiotherapy is given to the head this also causes hair loss. With radiotherapy, however, whether the hair grows back after treatment depends on the dose of radiation given.
The dose of radiotherapy used in the UKALL XII treatment is relatively low compared to that used to treat many other types of cancer. This means that usually the hair will grow back again once the programme of chemotherapy and radiotherapy has been completed.