It is indeed tuppence, or two pence.

As for Admiral Boom, while he could be a fey, he could also just be an old retired Hunter (of the Defender Creed).

In his old age, realising that he can't fight the forces of darkness any longer, he attempts to defend his part of London from the fey by promoting banality to keep the fey away, evident in his consistent firing of the cannon at the exact same time every day ostensibly for good timekeeping, but to promote order instead.
In a pinch, the cannon can also be turned against more mundane threats such as dancing chimney sweeps.