I may be misusing it in this case but, while masterfully used, the sets were all minimal and restrictive (aside from the garden scenery which was more of a backdrop it was all either in white rooms with little to no furniture and Old Amy's hideout) and it only featured the main cast + the Robots. It has more to do with holding back on budget than having it take place in one location, so I think it qualifies as the season had episodes that required many more effects and resources. From Wiki:
This definition can be applied to a lot of Who episodes though, lol.
edit: fwiw,
tvtropes also lists it as an example of bottle episode.
It isn't really a bottle episode, but it is a Doctor-light episode, which I would say is the New Who equivalent. Basically, they film two episodes at the same time, splitting the cast and crew across two different things at once. By having minimal use of certain members of the cast, it avoids clashing.
Love & Monsters (very little Rose/Doctor, carried instead by Elton & co) filmed alongside Fear Her
Blink (Martha has two lines, the Doctor has about 25 lines repeated on a recording, carried by Sally) filmed alongside Utopia
Midnight (Doctor only) filmed alongside Turn Left (Donna Only)
The Girl Who Waited (Amy/Rory focused, with almost all of Smith's scenes set on the TARDIS so they could be filmed in one day) done alongside Closing Time (no Amy/Rory except a brief cameo, Doctor only.)
These sort of bottle episodes have never been about saving money, but more about saving time. In many cases they've made the decision to tie these episodes to cheap ones, though. Like, Midnight and Turn Left both have some of the slimmest budgets of the series - but only so they could pour money into the big explosions and stuff for the Dalek episodes. They never divide their money 13 ways for a series, they pick & choose their battles. The nature of the episodes helps, as well - with the crew split, as well, it's really wise to dial back the scope a bit. I think that's a lesson learned with the awful Fear Her, which has a pretty traditional Who tone/scope - larger than all its other counterparts - and suffers on execution most likely due to a reduced crew.
So that's what they mean. Series One didn't need one, and nor did Series 5, as what caused the time shortage was inserting an additional episode (Christmas) into the schedule. Series 7 being split as it was meant they had additional time.
We'll probably not get another one of these episodes, as the indication is that Series 8 and possibly all series from then on will have at least one less episode to them, eliminating the problem.