If you are on an integrated course, then whether your FTO offers an hours reduction or not is entirely down to them. OAT, for example, will not offer any reduction, instead advising those with significant hours who do not wish to repeat the whole syllabus from scratch to perhaps consider modular instead.
However, the point I'd really make here is that if your FTO has made the statement that you need to revalidate to get the reduction, I'd see little point in arguing with them - it's unlikely they'll change their rules regardless of how unfair you see it as.
good luck!