Hi northUKbound,
I too have a same-sex partner and we literally just went through this process. I think most people would agree that as long as you are a UK Resident, which it seems you are, it would be much much much faster to go the route of DCF (Direct Consular Filing) while in the UK. It took my partner and I just over three months from when we sent in the I-130 to when he was issued a visa. They say 6 months or 10 months, but London seems to be much quicker. But it also depends on if you have everything in order. If you wait to file when you get to the US, it could take a year. Plus your partner wouldn't be able to join you for that (although I think there may be some visa you have to file separately to get them over but not sure on that one). All in all, it is much better to do it in London.
Your biggest challenge seems to be meeting the domicile and financial requirements. You have to prove that you are going to move back. That can either be by a job offer in the states, a lease agreement for a US apartment, or other strong ties. My partner and I got a house sorted before we filed so that was my way of showing I would be leaving the UK to reestablish domicile. The other problem will come with the financials. They are not interested in your UK earnings. You will either need to prove substantial savings or that your job in the US (meaning you have to get one first) will cover the minimum financial requirement. I believe it is something like 125% over the poverty line, which for a 2 person household is like $19-20,000 per year. If you don't have this, you can use a co-sponsor (like a parent) that will basically fill a form out to show their earnings are enough for their own household and yours. You also need to make sure you have been filing your taxes while abroad as they will ask for this.
You are correct that you have been married long enough to meet the indefinite green card and not the 2 year conditional one. However, as UK same-sex marriage has only been the law for a year, I assume you got your marriage elsewhere? Or did you convert your civil partnership? Just make sure you have the marriage certificate and not just a civil partnership. I'm sure you know that but wanted to make sure.
My final words of advice are, although UK Yankee forum is an excellent source for US-UK bound people, I would suggest using visajourney.com as well. It focuses solely on US bound immigration.
Good luck and feel free to message me with any questions as it is still very fresh on my mind!!! :-)