So, what action would you take if you went to fix a broken shower switch
but noticed it was wired onto an old wylex non rcd board.
Would you disable it or just advice not to use?

First of all...YOU WOULDNT TOUCH IT....advise client that new reg's demand it be fitted with RCD protection.........and offer applicable options.

Personally i would disconect ... make safe ...cos im like that :).........does that help ?

CMON where r u all.....................................?

Thank you for the advice.
I left it unusable, glad I did

good advice from e-lectrican.