I have never moved to the UK to study for a year or so, but I'd say I've spent roughly 6 months in London (mostly) out of every year for the last few years.
I would say personally not to bring many clothes except for jeans and shoes and underwear. (I also wear a lot of knits, so that makes bringing over 10 or more shirts pretty easy if I don't want to be buying things.) You can find plenty of discount stores in the UK for everything else, and even cheap winter jackets that you then toss at the end of the season. But good shoes are pricey here, you'll want your jeans that you know fit and look good on you, and of course you want the brands of underwear that are comfy on you already. I'd recommend shoes to walk a lot in, dress shoes that will look good with dresses and pants, and shoes that will handle rain and mud.
And I almost always take at least three pairs of those fuzzy socks in the winter. I've never been able to find the exact right ones I like in the UK.
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Space bags are great, tho for the stuff you do take. I've never had any problems with anyone cutting them open, maybe because I try not to pack anything that they'll feel they have to look at.
Mainly, the way I deal with it is to make sure I have my fiddle, my airline ticket, my laptop, my credit cards, my UK phone and SIM card, passport, and enough change for coffee, food, and such in the airport. After that, I can buy anything else or have it sent to me.