It's necessary for someone to apply for a 'grant of representation' (in the form of 'letters of administration').
Quote:
"The law sets out a strict hierarchy of those entitled to apply for the letters of administration in order as follows:
surviving spouse or civil partner;
children of the deceased;
grandchildren if their parent has died before the deceased;
parents of the deceased;
brothers and sisters of the deceased (or their children if the parent has died before the deceased);
half brothers and half sisters of the deceased (or their children if the parent has died first);
grandparents;
uncles and aunts (and their children if their parent dies first);
the intestate’s uncles and aunts of the half blood and their children.
Failing the above, the Treasury Solicitor can apply for the grant claiming bona vacantia on behalf of the Crown"
Source:
https://www.inbrief.co.uk/estate-law/types-of-grant-who-can-apply/