My story takes place in England about 1000 years from now, and due to the circumstances there is no modern technology. (Modern as in after the 1800s.) So of course there are materials and plants that were imported from other countries in the past, but I was wondering what materials would be able to grow there and what the clothing would look like//how it would be made. Thank you!
England has a long history of producing wool and linen.
Wool, of course, comes from sheep; England (especially the North Country) and Scotland have been sheep lands since at least Roman times. Sheep thrive well on land in that region where other crops don't yield enough to be worth the effort.
Linen is made from plant fibers, but unlike cotton, the parent plant (flax) doesn't depend on hot, dry weather to produce usable fiber; instead, flax can be grown in cool, wet climates -- a perfect match for England.
Cotton needs a warmer climate, though there may be varieties that have been bred or genetically altered to tolerate England's climate -- whether they've been "crippled" so they can't be grown without buying seed from the creators is another question. Cotton must have the seeds combed out before it can be spun; until the invention of the cotton gin in the early 19th century, the labor involved made cotton nearly as expensive as silk.
Silk worms depend on mulberry leaves, and I've just been reminded that mulberries don't tolerate freezing temperatures well. If the climate has warmed enough the winter frost is unusual in England, silk worms might have survived, but it seems unlikely (England is much further north than, say, New York or Minnesota, it's kept habitable mainly by the Gulf Stream). The larvae and adult moths are artificially sheltered in any case, but this is a case of needing to cultivate both the mulberries that the silk worms feed on, and the worms and moths themselves; if there was ever a time when society fell too far, it's likely the silk worms would have been lost.
Silk preparation is complex and labor intensive, even today -- the cocoons must be boiled to dissolve the adhesive the worm uses to bind them, then the thread unwound, multiple threads combined to make a thread heavy enough to handle in weaving, and the weaving is generally very fine, requiring specialized machinery to produce the cloth.
However, if silk worms are still kept, they produce another useful item -- silk gut. This is used for dissolving sutures in surgery, strings for bowed instruments like violins (where it's mislabeled as "catgut"), and has been used as fishing leaders (it becomes almost invisible in water and holds its strength for a good while).
Wool is sheared, washed, carded, spun, and either woven or knit. You've surely seen woolen clothing, as well as blankets -- many high quality overcoats and suits are made from it, as well as long winter underwear, socks, caps, heavy winter pants, mittens -- the list goes on and on. Wool is famous for the fact that woolen clothing is still insulative when wet, unlike cotton or linen.
Linen must be cut, the leaves stripped, the stems "retted" (essentially stored under water until the fleshy part of the stem rots away) and "scutched" to separate the useful fiber from the woody pith, then spun and woven. It produces a very durable cloth which, however, is extremely scratchy when newly woven; it was common, in the 18th and 19th centuries, to have a servant or slave "wear in" new linen clothing until it softened up. In addition to smooth-surfaced clothing, linen is traditionally made into bed sheets and table cloths, curtains and upholstery (because of its durability).
Further, the two are sometimes blended to make "linsey-woolsey" -- which effectively combines the warmth of wool with the durability of linen.
Animal fibers and skins would be a go-to solution for clothing; other foodstuffs might be in very short supply and reserved for human consumption, but as long as there's grass (which humans have to be pretty hard up to try to use as a bulk foodstuff), you can graze a herd of something whose hair you can shear for fiber, and/or that you can slaughter for a combination of the meat and skins. Both sheep and cattle have been common on the British Isles for millenia, and either animal will produce ample amounts of durable leather alongside its meat. Sheep will additionally produce wool, useful for insulation and for clothing in its own right. Certain breeds of goat are have been developed with longer hair suitable for shearing and spinning, and mohair, while a relative niche fabric, tends to be softer than wool.
Over into plant fibers, flax, whose fibers are used to make true linen, is common if not truly native worldwide, and has been used to spin thread and weave cloth for as long as 36,000 years, making it one of the earliest woven fibers in human history and one of the earliest known domesticated plants alongside staple grains. As such, you don't need a whole lot of technology to make linen cloth, you just need quite a bit of time (from harvesting the plant to wearing the shirt could take up to two years depending on relative level of technology and desired quality of fiber). The individual fibers, which can exceed three feet in length, also make a much stronger cloth than the shorter one- to three-inch fibers of cotton or of wool or mohair.
Cotton, which supplanted linen in most advanced economies in the 1800s, typically grows best in warmer climates, like the American Deep South, though the current "cotton kings" are in the South Plains of the Texas Panhandle, which definitely gets cold in the winter (but all the cotton's been harvested long before then and the fields sown with winter wheat). Prior to Whitney's invention of the saw gin for short-staple cotton, cotton was a luxury fiber, as long-staple cotton (ginnable with much older designs) doesn't grow as fast, while short-staple cotton required labor-intensive hand carding to remove seeds and hull fragments and straighten the fibers. While not exactly high-tech in its basic operation, most gins in existence today are machine-powered and computer controlled, and would be inoperable without a working electrical grid. Not to mention there would never have been many cotton gins in Britain proper, virtually none after 1980, so you'd be left to reinvent the technology from whatever you had laying around. Could be done, but you might as well spend that effort on processing a native plant like flax.