In June / July you do not have QTS and cannot be paid on the qualified scale. If you have a continuous contract across the summer, rather than a fixed term with a second contract starting on September 1st, then you must be paid at the qualified rate from the date you get QTS (sometime in August). There is overlap between the unqualified and qualified scales so it is possible for the school to pay about the same rate on UQ as Q. You need to discuss the UQ point that they are offering.
I am starting a job in early July, teaching MFL in a secondary school. However, since receiving the offer, I’ve been informed that until September 1st I would be paid as an unqualified teacher (amounting to about £18,000 rather than £24,500 in outer London, I believe). My mentor told me that I am lucky to get a job and shouldn't cause a fuss. Would it be reasonable to ask to be paid as qualified, especially as this wasn't mentioned until contract stage?