This is an extremely hairy area. I have been on the wrong side of it in the past. I am talking about intent to re-establish domicile. According to the manual it is enough that the US petitioner agree to accompany the spousal beneficiary to the US after issuance of immigrant visa.
But in practice, in the middle east and Asia CO's routinely use this as a reason to create obstacles and can really require you to jump through the hoops, including requiring the US petitioner to relocate first to the US and establish domicile before the immigrant visa is issued to the beneficiary spouse.
The CO ultimately has huge discretion on whether or not to make an issue of this. So, the fact that you are in London, and further if you are white, you may have no issue whatsoever. The NVC, is not the problem, as any minimal evidence of US domicile will satisfy them. It is at the consular interview where this may or may not be raised as an issue by the consular officer. Interestingly even sufficient liquid assets/passive income and property in the US will prove to be not enough if the CO determines that the US petitioner is in fact living abroad and must re-establish ties to the US before a spousal immigrant visa is issued.
So, if you want to be on the safe side, I would have US financial accounts, W2s, tenancy contract/mortgage, utility bills, voter registration, and if you have kids, evidence of enrolment efforts in school.
But if you want to take your chances in London, frankly I wouldn't bet against a successful outcome. Worst case scenario they will issue you a 221g and ask you to provide above things.