I would give it a day or two. BT (amongst other ISPs that use BT Wholesale) will update their system from BT Wholesale (who themselves query Openreach's system). If G.fast has become available within the last day or so then I do not think it is unusual for this to happen.
I could be completely wrong, but if it hasn't been updated within the next couple of days then perhaps someone could advise on how to resolve.
Out of interest, does it show available on the DSL checker when checking tel. number and/or address?
I understand that this is probably not the most helpful response possible or what you would like to hear.