See, I would assume the opposite and take two passport photos just to be safe--they aren't hugely expensive, you can get them all over the place. If you have time in the morning or now, I'd call around and find somewhere that can do them immediately and get them before your appointment, then no worries.
Congrats, by the way!! Hopefully someone who has done it will come along and be able to answer your question. We're more experienced the other way around here since people are coming to the UK, not as much the US. There are a handful though with spouses who got their USC before coming back to the UK, so maybe they can chime in.