Sunshine – I enquired about this today. You have to send a letter to the same address you sent your old passport to, outlining your application number and reason for cancellation. You won’t get a refund. Your documents probably won’t be returned quick enough (that’s what I was told – im travelling 4th April).
Also, the advisor strongly advised waiting until the old passport returns before going to a fast track appointment. Reporting a passport as lost, when it isn’t actually lost, could be considered fraud and getting caught doing that would arguably be worse than missing a holiday! Not what people want to hear I know but I think it’s the bitter truth here