Can you repair it yourself?
Weigh up if the mirror can be repaired (if it is glass only sometimes a new glass can be installed) or the whole unit (some just press back in to the mounting and you might be able to reconnect wiring) Glue it with epoxy? Get a replacement from a breakers? Get mobile again and put the whole thing down to another society failure. You deserve full credit for just working hard and trying to earn a living. I understand there are not many people interested in the UK doing this just now?
The change of job title
Notify your insurance company (online if possible to try and save an admin charge of circa £40) of your change in job title. That is no big deal but check the terms of your insurance cover to see if you are covered to drive to 'commute' to and from work as a single place of work or multiple places of work? If the latter you may need business insurance cover for your vehicle rather than the standard social, domestic, pleasure, fire, theft and commuting to & from a single place of work.
The mirror damage & the police
The effort you want to make to secure your damaged mirror is up to you. You can report it to the police but don't expect any action. When you can't get them to investigate assaults or burglary they aren't going to bother with a minor damage. They expect the insurance companies to do their job for them? They will probably give you a reference number and feel they have done their job at that. Also who is to say that the first impact of this guy's vehicle didn't break the mirror and punching it just removed an already damaged item. This is a 'fail to report' collision and driving without due care and attention rather than criminal damage. I would report this to the police on 101 and see what advice they give you. Keep it simple. Your vehicle was parked at the roadside and another vehicle & driver caused the damage.
If the police won't take action and you can identify the individual you can try and take civil action but you have to be determined. They probably have no insurance for their car as well. If the person who smashed off your mirror has diddly funds they aren't going to pay you and you will likely lose more. If you have their registration number you could make a claim against their insurance if they have it and get them to pay up. See if you have any legal expenses cover on your car insurance? Your own insurance company will probably 'write it off ' and you will end up suffering the financial loss with the excess. They will not want to investigate it as this is more expensive, it depends on the calibre of your insurance company.
Evidence you can collect for the police and your own insurance company.
a) Retain a copy of any useable cctv if you have it
b) Name & contact details of the witness - Get a summary of what they saw and what they can say (do they have a make and registration of the vehicle that the mirror damager was in and a description of that same person) The reg number is key but even then it might not be traceable. Drive round the streets locally it probably won't be far away if you are lucky. That same driver probably uses the same route on a regular basis, look out for them (similar time & date) and take any mobile phone footage if there is any encounter of the vehicle again.
c) Get your broken mirror and keep it
d) Lots of phone photos of your vehicle, the damage, the road, where it was at the time it was punched etc etc
e) Get a value of the mirror to be replaced
Finally - good luck