Can I apply for a Non-O visa extension and re-entry permit without confirmed flight details?

I'm going for my 12 month extension to a Non-O visa on Friday and then if all goes well will book a flight to the UK for a few days after so I can get the covid test done.

I'm planning to get a re-entry permit at the same time but on the form it asks for flight details both leaving Thailand to the UK and when returning to Thailand and I don't have outward flight book nor any idea of my return flight back to Thailand. Will this be a problem and if not can I apply for both the visa extension and the re-entry permit at the same time.

I'm panicking a bit (probably unnecessarily) as my family won't book the date for my mum's funeral until they are sure I'll be back.

many thanks.