My experiences are that BA and QF and Great Circle Mapper are often out by a few miles.

In this case, the airline making the determination on distance is BA since it was a BA flight which you travelled on rather than Qantas. On the flipside, it does mean that an award flight on that route is cheaper than it would be otherwise

Looking at Great Circle Mapper, LGW-KEF comes out at 1205 whilst LHR-KEF comes out as 1180 ( which is v close to the 1182 you were awarded )

Dave