When I first got my card I had a limit of £4k. I wanted to pay my tuition fees though and these were above the limit at £9k.
I rather sceptically tried to put through the transaction and it worked! Amex then sent me a text asking me to call them and the agent said; “There’s the available credit limit and the other limit we have that’ll allow transactions still to go through”. I was encouraged to make a payment quickly before it hits the account or a £12.00 fee becomes due for being over the credit limit
things may may have changed but I hope that helps!