Should I wait to hand in notice until the contract is signed?

The good news is - I have been offered a new job! The stressful part comes from my notice and informing my current employer… the timing is just before Christmas, and I am on holiday next week, I really want to at least speak to my manager and agree my notice period by Friday. I’m happy to share my formal resignation letter once this is all agreed slightly later just to get it in writing, but I want to let my employer know in person before I formally resign.

I have a preliminary contract from my new job, only missing a few personal details eg NI number which I have now shared. They haven’t signed it and nor have I, but I have accepted the job in writing and they have acknowledged this, all via email.

I think my new employer will then sign the final contract following my providing references, which I know I can get from my previous employer, I just want to double check with them to confirm they are definitely happy to do so and for me to share their contact details.

I guess my question is - while it feels like a safe bet, would I be stupid to have the conversation with my manager before a contract has been signed? Or, given the preliminary contract, written job offer, and my acceptance in writing which has also been acknowledged in writing, would it be safe to have the conversation tomorrow?