A great fighters workout...
Walking Lunges x10
Repeat for 30 minutes... or however long you can do it.
A couple of days a week do cardio that is geared towards using a great deal of energy over a short time, over and over again...
There a couple of articles in the general fitness section that may interest you.
This kind of training, the HIIT cardio, and now days...crossfit... is what's getting our US rangers and I'm sure your UK commandos into top fighting condition for urban combat.
Remember our own GymGirl from here at DF? She's in Afghanistan right now training some of our US military guys to be level 1 cross fit instructors. How cool is that?
Another thing to think about for a fighter is strength training. Train to get stronger, not necessarily bigger. Size will come from the heavy weight but you've got to be strong and fast...not just big. Train for explosive power and high output endurance. Not marathon or triathlete enduracne, but shorter time endurance where you are expending a lot of energy over a short time.
At best, I'm a mediocre kickboxer and NOT a groundfighter at all. Winning always came from drive to not give up, killer instinct (willing to actually hurt the other guy bad, then go kick his dog when I'm done), and from strength. Those things can carry you through even if your opponent is a better fighter and your mind is in a better place for it than his is at that moment. Even when sparring
with a world champion kickboxer who for some reason wanted to hurt me...or a state champion wrestler who wanted to hurt me...and actually had me beaten.
Lots of crunches too my friend...Gotta keep those abs strong.