Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
Ask the agent at your originating airport to re-book you so that you do not have any out-of-pocket expenses, if that is an issue for you.
I imagine that a Google search will yield a website for the company that provides LHR-LGW transfer service, and that website should indicate the forms of payment accepted for tickets.