I thought it might be helpful to put together a brief guide to where I tend to buy yarn in the UK, which shops I find reliable, which yarns I've particularly liked. Naturally this can only be a brief guide, we seem to be enjoying a time of abundance of yarn shops and yarn varieties.
First off, a guide to yarn weights, which can vary from country to country, a guide to substituting yarns, and the importance of swatching (checking that your gauge or tension matches that of the pattern so that it turns out the right size - not something to miss out).
I do not tend to use acrylic or artificial fibres, except in blends, as I prefer natural fibres, so I will focus on them. Natural fibres (wool, cotton etc.) do not have to be significantly more expensive than many of the acrylic yarns and I prefer knitting with them and wearing them.
Good all round shops
Modern Knitting - good for having its yarns organised well, sells a wide range of yarns from cheap basics to some higher end yarns; especially good for sock yarn. My particular favourites from here are Cygnet Superwash dk and the King Cole Merino Blend range of yarns, both of which are good quality pure superwash wool yarns.
Wool Warehouse - good wide range of yarns including the full range of Drops yarns, a Norwegian company, who are excellent value for money and very good quality natural fibres or blends. Drops have regular discount times when a range or type of yarns will be on sale, which are well worth looking out for. Also do a good range of German company Rico's yarns, which are at the good value end of yarn buying. My favourites here include Drops Alpaca, a light, soft, drapey yarn, one of my favourites ever, their other alpaca based yarns are good too; Drops Karisma superwash, another good quality superwash wool yarn and Drops Fabel, their sock yarn. Drops Paris and Rico Creative Aran cottons both deserve a mention as they are good for dishcloths and crocheted blankets as well as baby clothing and are very easy care.
West Yorkshire Spinners - buying direct from the manufacturer, this is a comparatively new find for me but I have just made three projects in their yarn one after the other and am impressed. Good value for money, I especially liked the Aire Valley DK, a wool/acrylic blend which is machine washable and uses British wool. I really want to try their newly released sock yarn, I have long thought that a British wool standard sock yarn was missing from the market so hopefully this will fill the gap.
MCA Direct - an established Scottish company who sell a good range of yarns including a wide range of Rowan yarns.
Pavi Yarns - has a good range of Cascade 220, an American brand of worsted weight yarn, often described as a "good work horse yarn", was for a long time Ravelry's most popular yarn, among other yarns and some good buttons.
Get Knitted - an established Bristol yarn shop selling a whole range of yarns, good for sock yarn, although their lack of stock control system (things appear to be in stock on the website but then turn out not to be) has put me off lately I will admit.
Stash Fine Yarns - good wide range of yarns, mostly big brand names, some good sales and discount codes, their bargain section is always worth a look.
Deramores - fast gaining ground thanks to aggressive marketing, stocks a wide range of yarns but are often more expensive than other shops for the same product. They have a lot of voucher codes which make the yarns more affordable, but on the whole I do not shop here much, but thought they were worth mentioning.
Addicted2Knitting - I should declare an interest as this shop is owned by a friend, particularly good for Zauberballs, good range of clover accessories and Knitpro needles.
Kemps of Sunderland - great for bargains, sell yarns companies are clearing out, really worth keeping an eye on
Black Sheep Wools - the other good shop for bargains and clear outs of stock
Sock Yarn Shop - does what it says on the tin, mostly sells sock yarns
Great British Yarn Shop - stocks a range of yarns including some exclusively spun for them, as well as being the sole UK stockist of popular US yarns Knitpicks
Purple Linda Crafts - specialise in crochet, great range of crochet hooks
Higher end shops
Meadow Yarns - a well curated range of interesting yarns from the everyday basics to true luxuries and exotic yarns not sold elsewhere. Also has the widest range of my favourite Knitpro knitting needles, I like the symfonies (the spelling makes me shudder), which are wooden. I would like to try the Hjertegarn Lima, a good value worsted weight wool.
Tangled Yarns - lots of lovely yarns, including the luscious Malabrigo yarns, they do a 10% off jumper quantities of yarn too
Loop of London - sells mostly luxury yarns from overseas, can be pricey, sole UK retailers for some US brands like Quince Yarns and Brooklyn Tweed
The Woolsack website lists retailers of British yarns far more comprehensively than I could ever hope to, but I will briefly mention:
Blacker yarns who sell rare breed yarns
John Arbon, an expert spinner with some gorgeous yarns
Maple Tree Yarns (again owned by a friend) for a wide range of British yarns
Jamieson of Shetland and Jamieson and Smith both specialise in Shetland yarns (amazing stuff)
New Lanark, brilliant value British yarn spun at the world heritage site of New Lanark mill, not the softest but very hard wearing
I hope that helps as a start, I will add to this and do a post at some point on my favourite independent yarn dyers. I accept no responsibility for yarn bought as a result of this post!