When to tell my manager I'm leaving?

I started my first CS role as an AO in HMPPS recently, but I have since been offered an HEO role.

The HEO role is still within HMPPS so I'm not even changing services, but it is a different department and location etc.

I'm waiting for my SC vetting to come through - I have no reason to believe I won't be cleared - and I'm hoping it will be the next few weeks.

As this is an internal transfer(?) at what point should I tell my current manager about the offer? They already knew I was looking for other roles and they're currently recruiting for more AOs so part of me wants to give them a heads up so they know they need to recruit one more!

Or should I be cautious and wait for clearance to come through before mentioning anything?