Getting a mathematics GCSE of at least a C grade is recommended before approaching larger companies or employers for training. Many employers will insist upon it.
An English GCSE of C grade standard or above is also recommended. This GCSE is essential so that employers can tell that you are competent in communication and literacy.
Other GCSEs are beneficial, but physics and information computing technology (ICT) GCSEs of C grade and above are recommended. Physics naturally aids in the repair and construction area of the plumbing business, and ICT is essential for dealing with computers when creating a bill, amongst other things.