I've never come across anyone who didn't return to work for three months post-maternity leave precisely to avoid having to repay their maternity leave.
However, looking online at the NHS employers handbook, it has this section:
Failure to return to work
15.82 If an employee who has notified their employer of their intention to return to work for the same or a different NHS employer, in accordance with paragraph 15.14, 15.15 or 15.17 fails to do so within:
i) 15 months of the beginning of their maternity / adoption leave, or
ii) three months of the end of their shared parental leave,
they will be liable to refund the whole of their maternity, adoption, or shared parental pay, less any Statutory Maternity, Adoption or Shared Parental Pay, received. In cases where the employer considers that to enforce this provision would cause undue hardship or distress, the employer will have the discretion to waive their rights to recovery.
I completely and utterly agree with you and could never consider doing that. Unfortunately, I have come across a general mentality amoungst some staff and members of the public that NHS workers should rinse the NHS for all they can, simply because they can!