So my car goes back after its 3 year lease in December. Does this mean I will now not get the £600 GCB?
No. Provding the lease of your car was taken out before 1st October 2022 you will get £600 split between £350 this month (if you give bank details) and then £250 when you actually return the car.
For future leases taken out on or after 1st October 2022, you will only get £250 at lease end.