He's wrong, you definitely cannot transfer it, and once it goes into BMW they will most likely do a key reading and record the mileage.
This is what I suspected.
I just need to decide if it's enough of a good deal at £18k to justify the increased warranty price.....I don't really need Emergency Service as I have RAC breakdown cover with my bank so the comprehensive warranty with a £250 excess is £63.28 which I guess isn't the end of the world.