Qilin

Oh my goodness, Elf -- thanks so much for the welcome, and for taking the time to give me such a thoughtful and detailed response! Yes, a lot of this was very helpful, and all of it was interesting.
The horrible attacks on Paris have convinced us that my husband should indeed come here (he lives in an English city that is very likely to be a future target). So it looks like I'm officially joining the VJ family. I've bookmarked the links you kindly provided and taken a look at the associated material. I know you said there's no telling how an individual medical will go, but I definitely feel like we'll be much more prepared when the time comes, thanks in no small part to your wonderful post.

It used to be yes, and then the new government took over.
US citizens are classed as non-EEA citizens, this means strict rules apply relating to financial test of means and there is no 'free movement' that would apply to EU specific citizens. Only the finances of the UK citizen are taken into account. There are no co-sponsors, offers from friends or family, or even specific living arrangements taken into account. The UK citizen must make £18,600 per year in a job that they have held for at least six months. For each child this increases a further £2,4000. The UK citizen can use savings, but this must be in the amount of £62,500 and held for six months. For each child this increases a further £6,000. You can do the conversion math on this and you suddenly realize the amount compared to the US is quite large. The statistics on this is staggering.in that a large number of Britons live on different means compared to those who may live in more wealthier economic areas. A large percent of the population would never be able to bring their spouse over to live. These rules have separated families and has been challenged multiple times, the latest will be heard in the Supreme Court. It has also made it more difficult for non-EEA spouses to even simply visit their UK citizen loved one. Many UK citizens immigrate to the US with the hope of bringing their spouse back with them one day, without realizing the difficulty. It's a subject of heavy debate.

Labels matter.
Your stepfather would be a fantastic JOINT SPONSOR. A JS does not need to be related to anyone - just needs to be a USC or LPR, and be over 18.
(No such thing as a co-sponsor.)

Sorry, my mistake. I don't have anything to add to a timeline yet. We haven't made any move whatsoever towards starting the actual process. I'm only seeking preliminary information so we can decide if I should move to the UK (a process we already understand fully) or if he should move here to the States.
Thanks for pointing me to the I-864. I had been looking at the I-134. Confusing for a newbie that they are both called "Affidavit of Support".
I appreciate your patience.