HSBC is lying to you; this has nothing to do with Safari and everything to do with HSBC coding their website to sniff and work only with Windows browsers. If I were you, I'd find a bank that actually knows what web standards are and fully supports them, and that obviously isn't HSBC.
A possible workaround is to enable the Develop menu (if it isn't visible already) from Safari>Preferences>Advanced, then go to Develop>User Agent and select a version of IE for Windows. The page will reload and with any luck you should be able to login. But if they're actually sniffing for what platform you're using, you're probably not going to have any success with that.
May the Force be with you.
Mulder