I'm reading this text about the statue and it says
The British Empire then was at its height and Boudica a patriotic heroine who died defending the liberty of her country against a foreign invader, whose power now had been eclipsed by its former province.
Reading that left me without words, I can't for the life of me see the logic behind it. Boudica to me represents a leader who defended her culture against invaders and therefore seems like an excellent patriotic symbol to use. My issue here is that Boudica lost and it seems the nobility of Britain (even in 1902) were the embodiment of the invading culture that persisted until that day i.e. the enemy of Boudica.
To me, the symbolism of Boudica really should have appealed more to the parts of the (then) United Kingdom that were still in touch with its Celtic heritage, especially Ireland at the time who was really pushing a home rule/independence agenda. Ireland was a rebellious island and was still defending its culture from what it perceived to be a foreign invader.
How on Earth could the nobility and politicians twist the symbolism of this heroine into their own ideologies? It would be the equivalent of Rome raising a statue of Hannibal.
Boudica was British, not Irish, so they were commemorating British patriotism when they raised the statue.
Your Boudica = pre-Roman = Celtic = Irish connection is quite tenuous and rests on modern notions of pan-Celtic nationalism that were not all that popular at the time.
It's true that the contemporary British rulers had little substantial connection to Boudica, but they didn't really have any substantial connection to the people she was fighting, either. The 19th century Victorian elite was a product of quite recent history - its real genesis was in the Glorious Revolution of 1688. But that is the nature of nationalism - it colonises the past.