I've recently watched Top Gear and one of the presenters said there: ...In fact, I was only doing a Victorian 23 knots. Why did he use the indefinite article when the noun is in the plural? And what does "Victorian" mean in this context?
Without seeing the full context, my best guess is that he was using the word Victorian as an adjective to describe something very old/antiquated.
Because he was travelling slowly, he wants to emphasise that his speed was like something very old, such as a vehicle from the Victorian era.
It sounds like it may also be sarcastic incredulity.
"My car is is so slow that it's practically Victorian"
This kind of thing is often done, for example:
Wow, this computer is so old it feels like it is from the stone-age
The stone-age is a period of history 10,000 years ago.