Which uniforms are you referring to? The majority of British uniforms from the Napoleonic period are fairly loose and relatively comfortable; most infantry wore the red tunic and 'comfortably loose' trousers, while Highland infantry wore kilts as well as the tunic. The shakoes (hats) worn by British infantry were redesigned a couple of time, from relatively heavy and uncomfortable leather ones before 1806 to felt ones (1806, redesigned to 'Waterloo' pattern in 1812) which better protected the wearer from weather and were both steadier on the head and more comfortable.

As far as cavalry uniforms go, they didn't need to be quite as comfortable for marching as the infantry, for obvious reasons, but were still practical. They were more ornate in some ways than infantry uniforms, but otherwise were similar: Red (or blue) jacket, breeches (they were tighter than trousers but still practical, think basically the 19th century equivalent of skinny jeans), a Tarleton cap or shako for light dragoons, a fur cap for hussars, or a steel helmet for heavy dragoons, and their weapons.

The facings on the cavalry jackets were actually less colourful than the infantry, being yellow, scarlet, blue, black, blue-green, and white, whereas the infantry had white, pale yellow, yellow, deep yellow, yellow-green, light green, blue-green, pale buff, buff, deep buff, orange, red, purple, blue, and black facings. These helped differentiate the regiments, but weren't particularly distracting or flamboyant.

Take a look at Mark Adkin's The Waterloo Companion for more information on British uniforms, if you're interested.

I'm on mobile right now, so I'll keep my answer restricted to British Infantry uniforms-

Contrary to what numerous authors have claimed, the British infantry's jacket was close fitting. Contemporary images universally show a tight fit to the jacket, trousers and breeches, and orders from the commander in chief actually remonstrated colonels for making their troops clothing too tight! However, with any given regiment having a number of ex-tailors in its ranks, it was easy to achieve a good fit in clothing. As I have written earlier, it is a curiously modern notion that tight fitting clothing must be impractical.

Fit is one matter, cut is another. The infantry jacket of the period may strike us modern observers as a curious garment in which to fight; however, compared to the long skirted coat it replaced, as well as the cutaway coats worn by other armies, the British infantry jacket was a highly practical garment. It buttoned with ten small buttons down the front, a three inch high collar and 3 and a half inch deep cuffs with four large buttons. Regimental facings adorned the shoulder straps, cuffs and collar, with corporals and privates wearing four inch wide loops of lace around the button and the buttonhole, tapering gradually to three inches at the bottom. The coat closed over the body, and was cut square in the waist, with short skirts behind.

Officers wore long skirted coats lapelled to the waist until 1812. At that time, all adopted jackets like the men, albeit retaining the lapels. Light Infantry officers had always worn short skirted coats.

Compared to its contemporaries, the British army was ahead of the times. The long-skirted cutaway coat was replaced in Great Britain in 1795, when skirts were shortened and lapels made to fasten by hooks and eyes to the waist. This garnent proved short lived- for the folliwing year the lapels were abolished and the coat made to fasten with buttons to the waist. The French, by contrast, persisted in wearing the long skirted habit until 1812, when it was replaced by a habit veste modeled after the Polish kurtka. Unlike the British, the French infantry continued to wear the bicorne up till 1807.

Woolen breeches and knee length gaiters were regulation legwear, but on campaign, these were frequently substituted for kersey overalls, and later by linen trousers. Infantry officers in the Peninsula adopted cavalry style booted overalls which buttoned on the outside seam, and fastened by chains under the instep of the boot. After 1812, grey woolen pantaloons for officers and trousers for the other ranks were authorized. Infantry simultaneously adopted grey gaiters.

Headgear- the leather and felt shako with its brass frontplate, cockade and plumes, strikes the modern observer as another impracticality. In light of the increasingly cumbresome cocked hat it replaced, the cap (as it was referred to in contemporary regulations) was a considerably more practical choice. In part, it owed its inspiration to contemporary Austrian casketts and continental Hussar mirlitons. However, there wS also no small influence from the round hat worn by troops in the east and west indies. The heavy and light cavalry wore similar watering caps, resembling the mirliton or bonnet a flamme of the Hussars. Again, these were worn in lieu of cocked hats, fur busbies or leather tarleton helmets.

Officers wore cumbersome cocked hats until 1811, when the false fronted Belgic shake was introduced. Prior to that time, officers had cut down their cocked hats into a less inconvenient, but also more unsightly, form.

Much has been written about the uncomfortable nature of the leather infantry stock. Light infantry at least discarded the encumberance in the Peninsula. However, it is relevant that old soldiers of a couple generations after the Napoleonic wars adamantly refused to give up their stocks-- even in the sweltering heat of Burma!

As a final note, the popular image of the British soldier in the Napoleonic period is of a fighting man utterly negligent in his appearance. This may have been true in the Peninsula, where Wellington cared little for niceties of military fashion (so long as his men. Did not resemble the French!), but was far from the case in other theatres. In North America, for example, officers were expected to follow dress regulations to the letter. We may tend to dismiss strict adherence to regulations as the mere fancies of pedantic martinets, but it must be remembered, in the words of numerous regimental standing orders, that when a soldier looked like a soldier- and not like a ploughman or a vagabond- , he tended to behave as such.

Much has been written on the subject of British Napoleonic uniforms, some good, most not. The best work I have found so far does not in fact concern the Peninsula or any other European theatre, but rather the War of 1812. Rene Chartrand's A Scarlet Coat is a superb overview of British military dress in that war, and contains much information relevant to the dress of the entire army in the latter part of the Napoleonic Wars. Good overviews may also be found in Michael Barthorp's British Infantry Uniforms since 1660 and British Cavalry Uniforms since 1660, Volume 5 of Cecil Lawson's A History of the Uniforms of thr British Army and W.Y. Carman's British Military Uniforms from Contemporary Pictures.

They were comfortable, held up to wear and tear, made it easy to distinguish units, and couldn't be easily altered by deserters. Very practical for their time.

You all should note that the slim-fitting breeches and trowsers were not like skinny jeans. Waugh recorded the authentic style in The Cut of Men's Clothes, 1650-1900. I ran them up plenty back when I did historical tailoring. Modern men wearing them would complain about how weird they felt because they were used to being tightly clutched around the pelvis by their jeans.

Period nether garments were designed for comfortably riding a horse, even civilian evening clothes. They are cut with no inner leg seam, only an outside seam, so that the basic piece is a Y shape that seems/seams totally alien. The back seam is much longer than we would cut, resulting in a non-restrictive baggy butt. You never see this when the coat is on. Also, the normal material was a knit fabric, like a wool jersey (jersey the stockinette fabric, not jersey the garment). This meant the legs could fit trimly without restricting. Think wrestling tights real loose in the caboose.

This same cut was used for bicycling suits in the 1890s.

So cuts were quite practical as well as proud.