I think being able to repeat a year is really useful and should be made more available.
My OH's family moved to a different region in at the start of third year (year 9 today), and initially spent a few months without a school allocation, then got allocated a crap comp in a different town, so learned nothing, and didn't actually get allocated a place in a decent comp in the new town until something like June so basically missed an entire year. Never recovered from that. Fourth and fifth years were a real struggle, ended up with a few poor CSEs. But he managed to claw it back at college and got a diploma with credits and distinctions.
At my crap comp, I was mercilessly bullied so ended up failing all my O levels and CSEs. I "re took" them in a different school's sixth form, passed enough, then went on to A levels there, so basically spent three years in sixth form and did my A levels in classes with pupils mostly a year younger than me.
It definitely should be something more readily available, especially when there are genuine reasons, such as moving and losing a year of education, and extreme bullying leading to moving schools etc.
Never really understood this obsession with everyone moving up a year whether they're ready and able or not. Moving through the classes should be done on the basis of ability, not age.