Well, the Celtic leader in the past was semi-mythological, so I'd rather a leader that led a major uprising that burned Londinium and Camulodunum even if she was defeated in the end. She's in the same category as Vercingetorix, who is the other actually existing figure who could be chosen.
Versingeteroix would have been my first choice - he might not have lasted long, but he did unify a much larger territory. He was meaningfully the leader of Gaul, while Boudicca only ever held sway over her tribe and a couple of neighbouring ones in East Anglia.
Not in the sense that Joan of Arc was, no. She did not lead troops (with questionable qualities or otherwise), and, as mentioned, Victoria was nothing but a figurehead; that an age was named after her is part misnomer and part irrelevant, as she also did not instigate any legislation (which is parliament´s job), nor did she do any heroic acts - in fact, all acts performed by her were rituals, as one would expect in a constitutional monarchy.
I'm aware of her issuing at least one decree that speaks to the values, at least, that Britain in retrospect wants to believe were cultivated in its golden age: upon learning of religious discrimination in India, she ordered religious equality be observed. Of course it had no binding political power, but it speaks of her as the sort of leader Britain would like to think of encapsulating its values (even though most of those values were likely inherited from her German consort).
So the only reason Victoria is a better ´leader´ than Marylin Monroe would be that she actually had something to do with government - albeit mostly by association. (Like Victoria having an era named after her, in Japan it is tradition to name an era after the ruling emperor - ignoring the fact that the emperor for most of modern history never wielded any real power.)
True, naming conventions aren't much to go by. We have an Edwardian era (albeit primarily referring to architecture and literature), but I don't think Edward VII is regarded as a terribly memorable or inspirational figure.
During her reign the modernization of English navy and development of a merchant navy bigger than before was well underway (it was in her time when the English navy abandoned old, unwieldy carracks and Great Ships in favour of small and fast "race-built" galleons, but also some heavier warship designs). This led to exploration and discoveries, which in turn led to colonies. Also, victory over the Spanish Armada (well, with much aid from Mother Nature) opened the seas for England. As England in Civ V is mostly a naval power, Elizabeth is a good choice to accompany this. Her father's naval ambitions were quite limited and the navy was full of obsolete vessels, save for a few then-modern Great Ships (although they also were too big for any reasonable action, even if they were armed with experimental breech-loaded guns. but even they proved to be unsuccessful and too difficult to use at the time).
I think this underscores how artificial it is to distinguish between rulers who "ruled" and those who simply presided as figureheads over major national events. Henry VIII essentially created the modern British navy; prior to that the island had not had a significant naval presence, but Henry was obsessed with two things (other than sport and wives): making England (and specifically him) credible in Europe again, with fantasies of emulating his predecessor Henry V, and technology. He introduced bronze-casting of cannon and, I think, also breech-loading to England. He designed what was intended to be the most sophisticated warship of the age, the Mary Rose. As in many things, Henry's ambitions outgrew his achievements, and the Mary Rose sank the first time it opened its gun ports, before ever firing a shot. But my point is that Elizabeth was not the innovator Henry was; she was simply in place during a process that was already ongoing.
As for the famous victory over the Armada, this is part of the national myth of the underdog Britain has built around itself, of the little island punching above its weight. It's never been true. Britain is one of the larger territories in Europe, it is so rich in natural resources that it was a favoured target for numerous invasions throughout its history and has always been among the wealthiest countries in the continent - from the most extravagant Dark Age burial sites in Europe to Napoleon's "nation of shopkeepers", to a country with the coal and metal resources to fuel the first stage of the Industrial Revolution. It's rarely been far from the main stage of European politics. For anyone sympathetic to Jared Diamond's ideas of the deterministic importance of geography, climate and resources on the way history plays out, the rise of Britain as a major power was essentially inevitable given its location, and happened several times under different rulers with varied policies.
The Armada is the same myth. What Britain likes to think is that it beat the odds, with grudging acceptance for the role the weather played (God, after all, being an Englishman). What actually happened is that the Armada was a fool's errand - the English had the larger and more sophisticated fleet, even without mishaps. It's likely true that the Armada would, if it had not been split up, ended Britain's naval ambitions for at least the immediate future, but it would have been as ruinous for Spain and would never likely have lead to the promised Catholic invasion of the mainland. Again, Elizabeth was there to be a figurehead and give an inspiring speech (or at least have one attributed to her), not to do any of the actual leadership.
