If the vehicle is your workplace then you should treat it as any other workplace. If you work in an office then you should expect conversations with family to be overheard and they may be recorded although you should be made aware that surveillance is in use if it is, a company vehicle shouldn't be any different.
If the vehicle can also be used for private use then you should be able to turn any surveillance systems off during private use, or at least have control of recordings made during private use.
There is a legitimate reason for recording audio on a dashcam, you can often hear an impact that can't be seen, it is good evidence. As long as they haven't installed the dashcam without your knowledge and hidden it so that you will not know it is there then it will be legal in a company vehicle being used on company business.
With any surveillance system, it's not the recording that is really the issue, it's what it is used for. They will need to comply with the data protection act and privacy law. If a recording of a private conversation was put on YouTube without your permission then there would be a serious problem and it would certainly be illegal, just listening to a private conversation is probably illegal - they should skip over it, if the recordings are never viewed unless there is an accident and then only viewed with your permission then there shouldn't be anything to worry about and no laws broken. If your employer is using them for monitoring your driving then they need to be very careful and you should certainly be made aware of what they are doing, but it is the same as any office job, employees can be monitored. If they are collecting personal data about you then it should be covered by the data protection act, what they are keeping should be registered and you should have access to it.