The exhibition was at Goldsmiths Hall in Foster Street. You had to sign in to attend the display, which I found a bit odd, I suspect I will get spammed to death now, but anyway ...
The Goldsmiths Hall is a "venue" with lots of old gold- and silverware on display. You can rent it out, apparently, and the organisers had rented the Livery Hall, which is an incredible sight and well worth a look if you ever have an excuse to see it. Not only is it sumptuously decorated and lit with wonderful chandeliers, but the stained glass windows are a wonderful record of the wardens of the Goldsmiths' Guild.
The exhibition itself was surprisingly small, occupying the tiniest corner of the Livery Hall. It consisted of merely 23 golden coins of various ages and denominations, and the aforementioned Double Eagle.
I have to say, I was quite disappointed with the showpiece of the exhibition - it simply doesn't look like five million quid on the hoof. However, the attendant was very knowledgable and was able to impart a vast amount of numismatic knowledge to me in a very short time.
Out of the other parts of the exhibition, the real "stand-out" coin (ha! ha!) was the "high-relief" 1907 Double Eagle, which apparently had to be "struck" five times to get the high relief. This made the coin very difficult, expensive and time-consuming to produce. It really does make for a beautiful coin though.
Another remarkable moment and discovery in London!