Hi Emma,
My sister travelled to the UK from Sydney with her daughter when she was 8 weeks old as her FIL was dying. Ella was fine.

The younger they are the easier it is, if you are flying TBH.
We flew longhaul with Sam when he was 7 months and he was a poppet. We flew again in January (20 months) which was MUCH more of a challenge as although he was good, he had to be entertained for a good part of the flight.
I was told babies should not be in infant seats for more than two hours. When Sam was tiny and I used to drive between Norwich to London, I never stopped, as it takes 2 hours. Must admit I hated any longer journeys though, so did my utmost to avoid them.
If you don't want to go then don't. Do whatever makes you comfortable. At least you know your DH will support you.