There's only three episodes in the entire series, so they need to get moving
Were only three planned? The UK system of doing TV shows seems to be similar to what in the US we would call a "mini-series" and not a full TV season. Typically a full US season will run 15-23 episodes with a new episode airing every week until the end of the season, then a gap of time, then a new season starts. Years ago it used to coincide with the seasons (hence the naming conventions

) in that there would hardly be anything but repeats aired during the Summer and then new episodes start airing in the Fall through Spring. Now, though, with the plethora of cable stations, each network has come to redefine what their seasons are. A mini-series, by contrast, are designed to be one-time events with only an X number of episodes that would ever be filmed & aired.
The upcoming Doctor Who season, for example, is only 8 episodes. For the US market that'd be considered a mini-series but with the UK it is a considered a "series" that will air, wait some time, then someday another series may or may not air. A few years back Patrick Stewart did the Eleventh Hour as a UK "series" and, of course, after I started watching it there were only four episodes ever done. No conclusion to the characters, no big origins story line, just four episodes.
