The "default" should be the old job but the employer can change this under certain circumstances:
From www.nidirect.gov.uk/sm/statutory-maternity-leave-returning-to-work
When returning to work after Ordinary Maternity Leave (the first 26 weeks of your Statutory Maternity Leave), you have a right to the same job and the same terms and conditions as if you hadn’t been away.
This also applies when you come back after Additional Maternity Leave (the last 26 weeks of your Statutory Maternity Leave). However, if your employer shows it is not reasonably practical to return to your original job (for example, because the job no longer exists) you do not have the same right. In that case, you must be offered alternative work with terms and conditions as if you hadn’t been away.