I tried to buy 20k of Dads new BMW using the Amex and pre-banked £20,ooo to the card then used that at the delarship. The delare charged me the 3% commission he was going to be charged but I thought it would be a good thing.
AMex BOUNCED the transaction despite £27,000 credit on the card stating that it was against the agreement to pre-bank funds. After a long debate with the authorisation drone I ended up paying with Switch so Dad could get his car.
How pleased was I (NOT!) to find out 5 days later that £20k had come off my card to BMW! The dealer was great getting the funds back to AMEX and the BA Amex team were excellent (as they usually are) in keeping me updated and they CHAP'd the funds straight back to my bank. It was nice seeing one months statement say I had earned 30,000 BA Miles! But that came off the following month!
If you have a good credit limit then you will be ok to buy up to the limit on the card but doing try to pre-bank funds or you will get much hassle!