Twice - both times the lack of brakes was because the rear cab wasn't properly shut down. The first time, I'd forgotten to do so myself (shut down = brakes max, reverser off, master key off). The second time I took over an AI service which was standing at St Pancras. On the approach to Stratford, I realised that there were no brakes aside from the emergency brakes. It turned out that the AI hadn't shut down the rear cab properly earlier. So always worth checking the rear cab before you start.
Yep, if the rear cab is still 'on' then you won't get any ReGen braking only Air brakes. Make sure all other cabs are Shutdown before setting off. You will know what cabs are turned on becuase it will show on the TMS.
Air brake (or rather electropneumatic braking) doesn’t work either. It only works when you put it into emergency. They should program it such that if any other cab is active, you can’t get brake release just like in real life.