Well aside from the specials year, the 2005- era has all been 14 episode seasons.
But the UK model isn't exactly the same as the US model when it comes to TV. Just because the 14 episode season works in the UK doesn't mean it'll work in the US.
Also in the case of DW, it's one of the most watched scripted drama shows in the UK. It's easier to justify the cost of producing the show if you are one of the best in your type of show (Drama), In the Top 10 for the channel if not for all channels. The DW Christmas special usually appears in the top 10 programmes of the entire year.
In ST's case by the end it was getting what 2-4m viewers in the US? As DW has already been mentioned, DW is getting about double that in a country with about a quarter of the population (UK)