Does my car really need a new brake control unit?

This sounds like the ABS unit. If the alert is showing it's a MOT fail. Unfortunately parts like these are not designed so they can be opened up and repaired. It is a high price, but if Volvo are paying 70% it's probably worth it to maintain the Volvo safety performance. I doubt you would get it anywhere else for less than the �400 you are being asked for