That's what I'm concerned about BA1321 - the fact that BA might treat me oddly because I'm throwing away the LGW-AMS leg. Hmm, still don't know what I should do then.

Pay the £80 and cross fingers? Guess if I miss it I'll have to foot the full-fare one way cost if BA aren't feeling particularly accommodating that day.
*crosses fingers and toes that inbound is on time*
EDIT: Btw, what is the transfer centre like at LGW number_6? I've forgotten as I haven't been through there as a transfer passenger in donkeys. Presumably it fine if I can wave my UK passport?