The Arco Britanico was a gift to the city of Valparaiso from the colectividad britanica. It was built in 1910-11 to celebrate 100 years of Chilean independence. The structure is covered in white marble and there are four friezes, one on each column of British 'heroes': O'Higgins, Cochrane, Simpson and O'Brian. We already know about Cochrane and O'Higgins (Chile's liberator). Simpson was another British Naval Officer/mercenary who was around at the same time as Cochrane (surprise, surprise-they didn't get along). Here he is:
I can't find anything out at all about O'Brian. I assume he was another naval guy who fought alongside the other three.
Anyway, here's the arch from sometime around 1925 and then from sometime last year. The photos are taken from different sides, by the way-the recent one looking east towards Viña and the older one towards the city centre.