You don't normally take on work experience in the first year but saying that the more you can do the better!
Im looking to go into the same area with medical / forensic photography.
Ring up the police headquarters in your area and see if you can go down. They may not neccesarilly do work experience but you can do a day visit and they can show you around the forensic department, if they like you they might let you go back again. I managed to get into the forensic department at Bridgend and sometimes it will go in your favour if one of your lecturers speaks to them first. You won't get any hands on experience as such but they will answer your questions and some may offer a work placement for graduates which is always worth keeping in mind so it's good to make the contacts.
Doing work experience in a morgue / coroners, again I'm not sure how easy it would be to do that, esp. in first year. If there is a teaching and research hospital in your area, they may well do work experience so you'd need to do a bit of searching around on the internet for that or your lecturers may already know who to contact. Applying to a regular hospital would be pretty hard and obviously post mortems are confidential and they don't just let anyone in student or not.
I wouldn't worry about it too much for now, get settled into uni first and then if you still want to, in jan / feb speak to your lecturers about doing some work experience over the easter break or reading weeks. If you get lucky like me, one of them may have worked in that area before and would be willing to help you out. The University should have contacts in those fields so if your lecturers are clueless then go to the careers department.
Again it's not neccesary in first year as you are only really learning basics, so it might be better to wait till 2nd year when you have a bit more knowledge under your belt.
Good luck!