Aww,, keep one or two of your favorite Hawaiian shirts,, you will at some point go on holiday in a warm climate and will be glad of them
(why not use them to brighten up your bedroom once you are here, hang them on the wall for color
)Also, definately keep your birks,,, we do have summer here, (it can get quite hot, and NO aircon! )and I wear my tevas all summer.
Wishstar gave some good sites to get an idea of what the girls generally wear here. You will be shocked to see girls dress very skimpy in the winter at night, with strappy shoes,, that still blows me away LOL,, but they are used to the cold.
I get alot of stuff at TheGap,, which you can still get some warm fleece things from them online. Depending on where you will be living,, people will wear heavy coats in the winter,, or like down south, where I live,, nothing heavier than a fleece jacket. You see alot of vest type jackets, worn over fleece tops down south where I live. When I am out shopping, it's a pain to have to take off a coat in a warm shop, so these vest things are great for that. They really turn up the heat in shops in the winter. It's not that cold in the South,, midwest American gets much colder. Get some long sleeve t-shirts, Roxy is a popular brand you can get where you live, that I see alot of girls wear here too, as well as Quicksilver. That way you have a bit of the 'beach culture' still with you.
Just keep to toned down colors, and you will be fine.
Dress black slacks are a must. That is almost a uniform here.
You have some time yet, so don't stress
another UK clothing site, that I shop at alot, is
http://www.dorothyperkins.co.uk/promostores/dp/index_ie.html They are quite reasonable costwise too.
http://www.topshop.co.uk, and
http://www.newlook.co.uk/, are fun trendy shops that are very inexpensive, and fun. Sounds like you will be at home with them, as they tend to be very youthful, and trendy.