If it's OK, I'd love to chime in about my experience. Recently got approved for the green card after more than a year (to be fair I did get 2 RFEs that delayed the process lol.)
My husband and I have been together for 7 years, I'm Singaporean, he's American.
Figuring out how to be together weirdly hasn't been a big factor because we spent a lot of our relationship in different countries where I was allowed to stay for 3 months, go back home, and come back again for another 3 months.
He's a teacher who teaches English in different countries, and money wasn't too much of an issue for me (lots of savings from 6-figure job.)
However the pandemic changed many things, we were apart for 16 months and truly done with not being together.
What I did in 2021 was come to the USA on ESTA, stayed for about 2 months, and then we spent 3 months in Costa Rica while he was interviewing for a job. He got a good job while we were there, and then I came back to the US.
During my first time coming to the US after more than 1 year, I got withheld and taken to the secondary inspection room (they didn't let me use my phone but I managed to sneak a text to inform my then-boyfriend about it) and questioned. I had to show them my bank account, which is a very big deal, as everyone knows, because they want to know you have the means to not leech off them.
The reason why I was questioned was because in 2019, I had extended my stay in the US to almost 3 months instead of my intended 2.5 mths. Even though the ESTA allows that, immigration doesn't like that.
So when I came back from Costa Rica, I very well knew I might not get back in, and I warned my then-fiancé and soon-to-be-in-laws. I had the flight out of the US printed out + my job ties to my home country. However, the officer refused to look at those, he didn't care, he just questioned me again about my intentions on staying. He let me in after.
My point is, if I were you, I'd prioritize time and finances, even if you're financially OK now. There can be a lot of unexpected costs and I would prioritize stability over anything else.
As in, I'd go and stay about 2.5-2.8 months in the US, get married on paper and start the process of sending in the I-130, Petition for Alien Relative. This is not an easy process because it's all your financial documents, proof of relationship, affidavits.... man.... it took 2 months for me to gather all of that and I had a lawyer.
Then you have to make sure you fill in the form EXACTLY or they will send a RFE to resubmit.
Then, go to UK/Italy like you planned, and start consular processing for the green card. I believe this is still how it works, anyone feel free to correct me.
Then he can visit, or come over for Christmas like you planned. He can then visit you again in UK/France March 2024... and then you'll both settle in Australia for a bit. By then things should have proceeded smoothly with the green card (as in, from what I know.)
If you're talking about engagement now, there's not much difference between getting married on paper now vs next year except time, so I'd go ahead and do that and plan the wedding ceremony for later.
For us, it was both a personal and practical choice, it just killed two birds with one stone. COVID already delayed us 2 years... for us long-distance folks, proper planning will help a lot because later you have to deal with things like social security, taxes, insurance, etc.
Also, there were unexpected health problems with us and with family... you just want to be able to handle all of that.
I'm very grateful to my past self that I've managed to avoid unnecessary hold ups and relationship problems due to financial stress. USCIS waived the interview, and I'm very grateful for that.
Right now, we're living in a very nice apartment complex in Las Vegas, with great neighbors, and I can properly be on my husband's medical insurance. My father-in-law recently had a series of strokes and we can go deal with that instead of worrying about the green card.
And yes, I would say that you being from the UK helps, it's more like Filippino passports that the USCIS pays more attention to.