Plain everyday cardigans.

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In a variety of colours but plain. Like the equivalent of a comfy pair of jeans, a cardigan you can wear with anything purely as a warm item, not ‘pretty’ or ‘fashionable’.

Preferably under £20.

Or, any specific shops to keep an eye out for cardigans later in the year?

It depends what size you need but M&S school uniform cardigans are good for plain black or navy. I wear a size 10-12 in ladies and their age 15-16 is generous on me and they go up to age 17-18. They wash and wear really well.

Also look at the men's sections of shops, the colourful ones are more likely to be on sale.

Unless you have literally hours/days/weeks to trawl those sites, the chances of finding something you like, of the type you want, that fits and is as described when it arrives and not a bobbly mess are low.

Plus get it wrong once or twice and you might as well have gone to Uniqlo or M&S and bought something new straight away without all that risk, because it's not worth returning stuff due to postage costs.

Uniqlo merrino and worth checking in on a regular basis, as they will discount when stock gets low on certain colours / end of season. You might want to look at Landsend, as seem to have big voucher code offerings off their stock. Don't foget to look at the menswear ranges too.