Unfortunately the only payment method is by credit card via Imperial College Students Union, and their site is truly dreadful; there is no security or privacy policy anywhere I can see, and a lot of the text on things like links uses tiny coloured letters that are almost impossible to read against the default background - and there's no way to change it. It simply doesn't feel like a professionally run site, and I really don't like the idea of entrusting it with more details than I've already given them - and if I'd realised how bad it was there's no way I would have even begun the registration process, since I've now given them (not the con organizers, who I hope are a separate group) my email address, and have to hope I won't get spammed.
This isn't meant to reflect on the con organizers, since I have to assume that the college won't let them do things any other way, but it's a real nuisance, and I suspect annoys a lot of the people who want to attend student events. I've told them why I won't be paying up front, and I hope that they'll pass on my concerns to whoever runs the site.