She states she will "maintain" and "enhance" workers rights

Yes, she does. However, Gove, for example, regards getting rid of the Working Time Directive AS an enhancement.

The Department for Exiting the EU has produced relevant factsheet
This states quite clearly "The Repeal Bill will ensure that the workers’ rights that are enjoyed under EU law will continue to be available in domestic law after we have left the EU. This includes rights derived from EU law, such as the Working Time Directive and the Agency Workers’ Directive. This will give certainty and continuity to employees and employers alike,creating stability in which the UK can grow and thrive."

Which is all well and good, were it not that the amendment to explicitly include these in the Withdrawal Bill lost, and that May seems very reluctant to say the "rights derived from EU law, such as the Working Time Directive and the Agency Workers’ Directive will continue to be available in domestic law after we have left the EU".