Breeches became longer — tightly-fitted leather riding breeches reached almost to the boot tops — and were replaced by pantaloons or trousers for fashionable street wear. Coats were cutaway in front with long skirts or tails behind, and had tall standing collars. The lapels featured an M-shaped notch unique to the period. Shirts were made of linen, had attached collars, and were worn with stocks or wrapped in a cravat tied in various fashions. Pleated frills at the cuffs and front opening went out of fashion by the end of the period. Waistcoats were relatively high-waisted, and squared off at the bottom, but came in a broad variety of styles. They were often double-breasted, with wide lapels and stand collars. Overcoats or greatcoats were fashionable, often with contrasting collars of fur or velvet. The garrick, sometimes called a coachman’s coat, was a particularly popular style, and had between one and three short capelets atached to the collar. Boots, typically Hessian boots, already a mainstay in men’s footwear, became the rage after the Duke of Wellington defeated Napoleon at Waterloo in 1815. Wellington boots, as they were known, sported low cut heels and tops that were calf-high.
Lord Grantham is also wearing pants--you can tell because his boots are low enough that the ends of breeches would show, and the buttons would be noticeable. I want to point out another style shift: see how the cutaway of the coat has changed shape? The earlier upside-down U shape has been replaced by a squared-off waistline and straight angles on the sides. Gradually, the straight area would become wider. It was not long before tailors, probably influenced by this new straight shape, began cutting a waist-seam into coats. That seam got rid of the slight wrinkles that tended to gather around the waist and is a hallmark of later Regency tailoring.
Here is Mr. Darcy's version of the cream-colored waistcoat and neckful of linen. Note that his waistcoat has self-fabric buttons - the only exception to this I have ever seen in period waistcoats are thread-wrapped buttons, never metal, shell, or other materials. I'll add that they did an excellent job with the hairstyles in this film. He looks as if he's straight out of a painting.
Having gone over these daytime styles in detail, I want to take a moment to look at more formal styles. Much of what goes above will be applicable, but there are a few important differences and I think it's useful to see them side by side, as here. Here we see "full and half dress for April," circa 1809. If full dress is a tuxedo, half dress is more like a business suit. And like a tuxedo, Regency full dress retains "fossilized" details from the period before. The bicorn hat went out of style for daily wear quite early in the Regency, but it remained de rigeur for full dress - probably not least because it could be tucked under the arm, unlike a top hat (see image below). Also, the shoes have decorative buckles on them, which is a very eighteenth-century detail. Fancier fabrics were used and I've noticed that for some reason, full-dress outfits usually include a white or off-white waistcoat.
Colonel Fitzwilliam is sporting a style very similar to the above, particularly in the cut of the jacket. The contrasting velvet trim is something that became more and more popular into the late Regency. The only thing that seems a little odd to me is the pointed detail of his waistcoat. Generally the cut of the waistcoat was straight across, and only about two inches showed, not four as here. Still, the feel is captured, down to the hairstyle and the gloves, hat, and cane.