Good Will, thrift stores, garage sales, and people selling their clothes for cheap on Ebay. That's pretty much the only places you can buy super cheap stuff.
You can modify the stuff you buy there. There are millions of tutorials and how-to's online from simple sewing basis, dying jeans, to more complex things. Christ, you can buy an ugly pair of blue jeans at Good Will for a couple of bucks and turn them into skinny jeans in your favorite color. Search the whole clothing section, go to different stores in other places by you, you can find tons of things.
Look up on youtube how to screen print yourself, the library might have books on it, take maybe 5 minutes to learn how to make a stencil, get some paint or fabric markers and make a print on a t-shirt with stenciling. Seriously, take advantage of Google and Youtube to learn how to make things.
Abercrombie, Aeropostale, Hollister, Pac Sun, and American Eagle are stores. Websites-
www.abercrombie.com (you can also try kids if you're young enough)
www.aeropostale.com (p. s. is their line for kids 7-12)
www.hollisterco.com (dudes is boys, bettys is girls)
shop.pacsun.com (they have swimwear too)
www.ae.com (american eagle)
I think they all ship to Great Britain, but you'll probably have to pay an extra fee to get it shipped there. Have fun online shopping!!!