I’ve found the rule about divided races:
Subject to the provisions of paragraph 5, the following types of Race are permitted to divide:
Weight-for-Age Races in which the Total Prize Fund is;
£15,000 or less
The only races on that Southwell card that have the provision for being divided in the race conditions, are the three class 5 handicaps, coincidentally each worth £15,000.
However, paragraph 5 does contain a get-out clause:
The BHA shall:
after Closing, determine an order of precedence for the division of Races permitted to divide