Oops, I forgot to mention, you will need to sign up with two platforms. (BTW, Box trading isn't my personal technique but a FX trader favourite!)
I use IGindex and GGmarkets. Go long on one platform and short on the other. Also bear in mind, you can apply the same technique on shares too! (The problem with buying shares using a broker platform is you cannot profit on the way down, but with Spreadbetting trading platforms you can! - plus all profit is TAX-FREE! )
www.igindex.co.uk
www.ggmarkets.com
if the box technique is not how you trade, how do you trade then?