TheScullster wrote:
Talking to one of my colleagues who is currently in this situation he tells
me that the contractor doing rewiring works has stated that he must rewire
the whole house because otherwise there will be a mixture of cable
specifications/colours which is not permitted!!!!

Nonsense, you stick a label on the consumer unit stating
"This installation contains mixed standard wiring colours"
and record the fact on the installation completion certificate.

However, with a flooded house, I would be tempted to replace
all fixed wiring anyway. Even if not immersed, you never know
where capillary action could have taken the water.

--
JGH