I would second the T5 left luggage by M&S - very professional and reasonably priced.

As you mention - no electrical things in the US Embassy at all, so worth doing the pre-security pat down ritual when you put your bags into left luggage (no phones, headphones, etc, etc.) - having been through the same scenario (BTW the GC 'interview' is about as rigorous as that for global entry - 2-3 minutes of the guy checking the documents and little conversation!).