The exact date when the story is set is never given; generally, the movie's taken to be set in 1813, when the book was published. I'm not going to go into the full story of English dress in the early 19th century because I'm planning a post on that later this year for the Historical Sew Monthly - specifically, what really went on with English and French dress during the Napoleonic Wars? - but I would say that that's reasonable. 1811 to about 1816 as a range.
|All caps from cap-that; could not resist using this one|
|From La Belle Assemblée, for November 1813|
In the scene captured above, Mrs. Bennet and Mary are in morning dress, while Kitty and Lydia are in some version of evening dress - Kitty's is plausible for a less formal gown, but I don't think anyone would have made evening dress with a print like Lydia's. While there are scenes that feature chemisettes and fichus, there are numerous outfits where necklines are just too low and bare for morning dress, mostly for Elizabeth. Of the Bennet girls, only Mary tends to wear gowns that come up to her neck without a chemisette, and overall these high-necked gowns (unattractive to a modern eye) are worn by characters who are either unsympathetic, like Mrs. Hurst, or somehow at odds with Elizabeth, like Charlotte after her marriage. Though this is all less obvious than P&P05, it's still an example of artistic inaccuracy used deliberately to illustrate characterization.
|He has threatened to dance with us all.|
|Charlotte Sparrow, William Owen, ca. 1815; Staffordshire County Buildings Picture Collection PCF 6|
Another small but frequently recurring issue are the shapes of the necklines on very many dresses, especially Elizabeth's.
During the Regency, there were two common necklines: cut-in and constructed. The cut-in neckline was, like Elizabeth's above, a curve cut into the front of the bodice; the constructed neckline was created by adding straps to a rectangular bodice front. The cut-in neckline was more common in the 1810s and 1820s, while the constructed neckline was more common in the 1800s and 1810s. And here is the most important aspect: the cut-in neckline was usually shallow and wide, the constructed neckline deeper (but also wide).
This ahistorically deep cut-in neckline has become very commonly used in Regency-set period drama, sometimes a little shallower or more pointed than this. I should note that a rounded, deeper neckline is characteristic of the Neoclassical period - but this is because the front-closing drawstrings very frequently used at that time created it. Once the front closure was lost in the early 1800s, this shape fell out of favor. The desirable silhouette was broad through the shoulders and long and narrow in the body and legs, with a very high bust, and the deep cut-in neckline de-emphasizes the width and height of the bodice.
Elizabeth's dresses all have this neckline, and in general all seem to be made from the same pattern: the bodice gathered slightly at the bottom of the neckline and more at the waist, that waist being just a bit below the bottom of the bust. (This gathering is not, strictly speaking, inaccurate, but it was more common for cut-in necklines to go along with darts around 1813. I'm also not sure I've ever seen a gathered cut-in neckline around this time, either - there are 1820s examples, however.) While most dresses of this era do have a great number of similarities in construction, I don't believe it would be normal to have all of one's dresses made on the same exact lines. For one thing, people would not purchase and get rid of all of their clothing at one time - just as today, they would gradually replace clothing as it wore out or became out of date, and a dressmaker would be unlikely to duplicate a gown she'd made a year ago exactly. Gathers would change to darts, a neckline would be a different shape or height, skirts would have more or less fullness, etc. A woman wouldn't own only back-buttoning gowns, as Elizabeth does, but would have some that gather on ties, lace, or fasten with hooks.
My last point is more general than specific. One thing that stands out to me is the volume of curls around many characters' faces.
Elizabeth always has a thick mass of curls on either side of her face, from a few inches away from her center part descending almost to her jawline. Jane's curls are similar but much shorter and more defined; Mary has only a few very small ones, as does Kitty; Lydia, like Mary, tends to part her hair on the side, but has many long curls all around her head and incorporated into her hairstyle. Their mother has two very large and very regular bunches of short curls, and Charlotte tends never to have any curls at all. Caroline has a hairstyle very similar to Elizabeth's (as does Anne de Burgh) although her curls are more like waves, and Mrs. Hurst has single short curls placed all around her forehead.
|Eliza Augusta Falconet Middleton, Nicholas-François Dun, ca. 1812; Gibbes Museum of Art 1960.10.4|
It's very common in movie costuming to dress older or more conservative characters in older styles, often exaggeratedly so, in order to emphasize the disconnect between them and the main characters. Sometimes wealthier or more fashion-conscious characters wear clothing which would not come into fashion for several years to make them stand out. The interesting thing about this case is that the actual dates of the hairstyles are not relevant in the costuming: it is just about the effect. Mrs. Bennet's hair looks dowdy, despite being too fashion-forward - and styles with no or few curls, actually appropriate for 1813 or a little earlier, appear childish and unsophisticated. (Mrs. Hurst's, I would say, is meant to look conspicuously dressed.) Elizabeth's hairstyle works as an attractive balance between dowdy, childish, and fussy in terms of its visual effect - youthfully natural - without its actual date being relevant to its interpretation.
The 1995 Pride & Prejudice is remembered as a highly accurate production, and so it should be: there are some costumes that are very good, and a lot of attention to detail throughout the production. But it's not without its flaws, and it's important not to let its visuals supersede extant garments and other primary sources when researching.