There's a very good reason the trains can't stop at Wembley Central, platform length! It's an H&S requirement for trains with automatic doors to be fully platformed, unless there's a safe way of working with SDO (selective door operation). In the case of the Avanti fleet, SDO can only be used at locations where fixed infrastructure (called balises) are installed. Balises are basically bits of lineside kit that send a message to the train's software to keep certain doors locked when the rest of the doors are released.
On the Avanti Network, they are only installed at Haymarket (Edinburgh), Lichfield Trent Valley and Manchester Piccadilly platforms 13 and 14 for diverted services.