MEClarke

Absolutely file as soon as possible. There is a MASSIVE backlog due to the embassy shutting down for months for COVID, and things have slowed yet again with the lockdown. We first applied 13 months ago and have our interview next month. People applying now will likely face an even longer wait with the backlog and reduced processing power. 
 
You can always wait to submit your docs to the NVC at that stage. Also, the visa is good for 6 months from issuance. So worst case scenario, you get the visa and then stick around the UK for another six months (though I would enter the US sometime during that time just in case). 
 
Also, I’m from MA with a UK husband too! Maybe we’ll see you around!

You file your I-130 petition to the US address. Stay in the UK. Times are so hard to predict with Covid slow downs and shut downs. There are people interviewing in London in March who filed their  petition anywhere from late 2019 to maybe April 2020. TheLondon embassy appears to be doing interviews just a few days a week with reduced staff. If/when things open up more, yours could go faster or get bogged down when London starts interviewing fiancés and tourist visas again. There is a huge pile of those who got paused almost a year ago. 
 
if you start now, you can always delay and reschedule once your file makes it to London and you’ve been assigned an interview date. You let that date pass and request a new one (as explained on the embassy website.) You can also be slow sending things to NVC if it appears you got there way too soon. 
 
i wouldn’t worry about domicile. London is very easy on that one. 

I would file ASAP. A year is a good estimate without all things COVID going on. If things speed up dramatically then you can always pause it at the NVC but I really don’t see that happening with the huge backlogs at the embassies. Most visa categories have been suspended during COVID so when things open up the embassy has a huge backlog of cases to get through plus the usual workload. 

You should file now, hopefully his Visa will come through in that timeline.

My wife (US Citizen) and I were in the same position, she has been living here with me on a spouse visa since 2017 so that we can be together whilst I complete my military service. We filed our I-130 back in March with the aim to move early in 2022, back then things were taking a lot longer to get approved than they are now. The longest wait now seems to be after NVC approval to interview in London!