I’ve just had a look at the BHA website.
Under code 12, the Stakes & Prize Money Code, section 17 states:
“If a Race has fewer declared runners at the ‘Off’ than the number of placings with prizes, the surplus prizes shall not be paid.”
In addition, section 18 states:
“After a Race is declared ‘Off’, any money or prize which is surplus due to fewer finishers than placing(s) shall be distributed to the finishers equally.“