In her book British Animation: The Channel 4 Factor, former Channel 4 commissioning editor Clare Kitson relates how she first saw The Simpsons in 1990 and realised the potential of adult comedy animation. "My own enthusiasm to commission adult series stemmed from a recognition that it could be a brilliant genre," she says. "I also hoped that adults, unused to watching animation of any kind, might come to it via comedy series, discover its many virtues and so think of giving the auteur shorts a try."
In the latter half of the decade a slate of three adult cartoon series appeared on Channel 4: Crapston Villas (created by Sarah Ann Kennedy and premiering in 1995), Pond Life (Candy Guard, 1996) and Bob and Margaret (David Fine and Alison Snowden, 1998). Crapston got good ratings, while Bob and Margaret turned out to be more popular in the US than in the UK and Pond Life was scuppered by poor scheduling (read Kitson's book for the whole sordid story); all three were strong series that received good reviews. Well, none of them had the longevity of some of their American counterparts - Pond Life and Crapston Villas both lasted two seasons each, while Bob and Margaret mustered a more-respectable-but-not-exactly-up-to-Simpsons-standards four seasons - but that's British TV for you. Let's not forget that Fawlty Towers only had twelve episodes.
Later, in 2005, came Bromwell High, which was a flop: "it notched up barely half a million vewers," writes Clare Kitson. "This failure signalled an end to peaktime, expensively-animated narrative sitcoms."
BBC and ITV have also commissioned adult cartoon series, and it's interesting to note that almost all of them have been sketch comedies. There's 2DTV, Monkey Dust, Aaagh! - It's the Mr Hell Show and Headcases; Creature Comforts could also be included in the list as a mutation of the genre. I can think of only two notable series, Stressed Eric and Rex the Runt, that aren't sketch-based. Ironically enough, Channel 4 rejected 2DTV when it was pitched to them precisely because it was a sketch show and not a traditional narrative sitcom.
Blind Justice. See this post for more information on the series.
One thing that's worth noting before I close is a body of work that generally isn't included in lists of animated TV series yet, technically speaking, fits the bill. I'm talking about the likes of Lip Synch, Blind Justice, Sweet Disaster and Animated Minds, series that fall outside traditional TV show formatting and are instead usually regarded as packages of themed short films. Generally speaking each short is handled by a different director using a different technique, and there are no recurring characters or settings to link the shorts, only a broad theme (mental illness in Animated Minds and feminist criticism of the legal system in Blind Justice, for example). This unusual approach to making an animated series has recently been adopted by Hollywood: with the help of Japanese studios, Warner Brothers has been producing similar sets of themed shorts, albeit ones with a far more commercial leaning (instead of social issues, they take their inspiration from The Matrix, Batman and the Halo video games).
On a similar note most of the series discussed in this post have short running times - ten or fifteen minutes an episode. This may at one point have been looked upon as a disadvantage when compared to half-hour series from America, but seems far more acceptable now that short-form web series such as Homestar Runner have demonstrated that less can indeed be more. Perhaps these adult animations were ahead of their time.