On Just Giving you can change the currency by clicking on USD and selecting GBP. It really is easy to use.
The good thing about using Just Giving, rather than using Paypal Direct, is that you can automatically Gift Aid the donation, adding an additional 20% onto whatever you're giving.
I think Just Giving are having a few issues at the minute though, their site appears to be a little unstable. I'm sure it'll settle down in an hour or so.
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