I have just entered both my 10th year of Law Enforcement. (patrol and School resource have been my two assignments) and 20th year in The Martial Arts. (karate) Karate has quite frankly not been of as much use to me as high school wrestling has been.
I was just at the Police academy last week. I am workout partners with the DT instructor at the police academy. I will tell you all they teach now is joint and ground fighting.
They taught us kicking and punching (hard hand tech.to be used just before deadly force) as well as wrist, joint locks and take downs. (open hand tech. also the most used in the real world of LE). In 10 years the only strike I have used is a block. We have evolved into a world of liability. Everyone is Law Suit scared. so they come out with tools Like the Taser and Asp. that will back their tool as long as they are used correctly. My advise to you would be. To train what you enjoy no matter what it is. They will teach you what they want you to learn, then tell you to use it only after your Taser, Pepper, and Asp baton does not work. Good luck with Law Enforcement it is a fun job. I don't know where you live but here almost everyone makes more than cops, and I work at a good Department that pays well compared. It is a calling not a job. I would say that if you want to train TKD go for it. But BJJ and Aikido type style would be the most useful. in this day in Law Enforcement.
Also Don't get caught up in that we train Law Enforcement Crap. everyone trains the police or military of some type. I know cops that train in yoga and Tai chi does not mean it would be effective in the streets as a cop and then again it works for them. People just use that because that gives them some validity, but really I think most arts can say they trained the police.
To repeat myself for about the 19th time in this reply (i need Sleep) Just learn what they train you in School. then when you get out of school. someone will tell you to forget everything you have learned and they will teach you the street way to police.