It's been given the OK because of their 'protection' they offer, such as not saving any numbers longer then 3 digits and anything that contains the @ symbol.
They track you via a unique number instead of a name but this doesn't stop them finding out this information. I.e. I buy something on a forum and send an email containing my name and address. They then have this data, stored in the US and no longer under our 'protection' laws and so can be sold on further. It wasn't reviewed by someone who knows much about technology obviously.
Sorry, no there isn't a way for people outside the UK to sign it. Although this doesn't effect me personally, both my internet reseller and wholesaler (Aquiss and Entanet) have vowed not to sell to Phorm I do feel quite strongly against this being implemented.