If you get an instruction "line up and wait" and you respond with "G-ABCD holding" then, technically, you are refusing a conditional clearance. No problem with that, it is your right as PIC and you don't have to provide a reason for it.
What you obviously should NOT do is enter the runway anyway without further instruction to do so. If you promised ATC to hold, you should continue to hold.
Last edited by BackPacker; 29th November 2007 at 22:46.
Reason: Deleted some stuff that was not relevant