Kate is a member of the Labour party.
This means Kate will vote in line with Labour goals and principles.
In return the Labour party has provided staff and funding support for Kate's campaign.
Kate may not always vote in line with Labour goals and is in principle free to "rebel". However, rebelling regularly will likely result in removal from the party, and so the loss of benefits associated with being their representative - so it should be a rare event.
The Labour Party Principles
We want to build a Britain that works for the many, not the few. That means building the homes we need to rent and buy, keeping our communities safe, giving our schools the funding they need, and restoring the NHS to its place as the envy of the world.