Stephen Hawking's ashes were recently interred at Westminster Abbey. Normally monarchs and heads of government are buried in the Abbey.
Who decides who can be buried at Westminster Abbey?
Does the person to be buried have any say in the matter? (If they give other instructions before death, can this be overruled?)
Is there a formalised process for selection? How does one nominate a person to be buried there?
The decision is made final by the Dean of Westminster. The honored is given to royalty, aristocrats and other people of note. The specifics of nomination I am not sure. Most likely a discussion between the Abbey and the Royals. For some I would think it is assumed but other are most likely offered either in advance or the family is given the option. I would think someone with enough notoriety would be approached about the option in advance to avoid problems post-mortem.