It's all online and they already have it. But it's an extra level of confirmation in case they require it. I never rely on technology and printed mine.
I put where I boarded and started my journey, UK. The connecting flight was delayed and flight number changed so it would have been pointless to put that, especially as I didn't start my journey there. It was accepted as UK and I'll stick to that next time too.
Why are you trying to fill it in outside of the 72 maximum? It clearly says not before 72 hours. It's a few minutes task with very little to fill in on it. You can even do it in the airport, the plane or on arrival.
First year extension the 800,000 has to be in bank, whilst you do the 12 months 65,000 transfers. Then can switch on second extension. Think that's the way.