You can only order a new car 12 weeks before your current lease end date, Motability write to you when it is time.
With the last couple of years delays (WLTP, Covid etc) the wait times for new build cars has been almost double the 12 weeks but as usual Motability is always so far behind the curve that by the time they allow you to order with 5-6 months left on your lease the delays will have passed!