I think the 'completion' bit is to do with the portion that must be done in a JAR country ie the UK.
As for the question about "...do I need all these things?"
A flying instructor rating isn't 'needed' in the sense of 'Is it a mandatory qualification to be hired by an airline'.
HOWEVER, the catch is that it is very rare to be hired by an airline with minimal experience. A FI rating is often used as the pathway to gain a more 'hire-able' experience level. In that sense, yes, it is needed UNLESS you have an alternate way of gaining those precious hours.
Student pilots must understand that the qualification you gain is an ENTRY LEVEL ticket ie it is the bare minimum qualification that would allow entry into the job. It is not some sort of guarantee.