My neighbour had a flood a few years ago, and although the water didn’t run through the CU, just the moisture in the air was enough to damage the breakers.
This didn’t stop the elderly father of another neighbour from turning on the power to give them lights…. Even though water was visibly running down pendant flexes.
First thing I did after the homeowner demanded his insurance company had me do the electrical repairs… was to install a temporary board for dehumidifiers.
I tested every length of cable, and only had to replace metal back boxes, new faceplates, new cabling for additional smoke detectors and a new CU at the end of the job.
Although he had a very good electrician… the joiner, plumber and labourer were idiots.
3 core between two way switches on staircase caught with a plasterboard screw. 3 separate water leaks which resulted in another new heat detector… and a gas leak!
Story over. There is no 28 day limit… but the installation needs tested before energising.
If in any doubt, a temp board can be fitted as soon as possible, leaving all of the installation not energised.
Even if the boiler wasn’t affected, damp could damage the internal board…. Or water got into a remote part of the heating system like a room thermostat. Best to leave it off too.