When Henri became Grand Duke, all the heirs to the European thrones were invited. Only Charles declined to attend.

I think Guillaume's enthronement will be the same - invitations will be extended to the heirs, along with invitations to the monarchs and spouses of the Netherlands and Belgium (The three Benelux countries have a very close relationship).

I would be very surprised if, in the event that William declines his invitation, that the Uk was told to just sent whoever was available.