The main plot of the story is that all the different special forces (SAS, Royal Marines, Delta Force, GSG-9, etc.) are fighting aliens made from a sort of nanotech grey-goo. The alien goo is constantly learning and adapting to the soldiers weapons and tactics so the war is getting harder and harder. The soldiers all have lots of augmentations (cybernetics, etc.) in order for them to keep up with the aliens.
There are 2 books in the series - and I feel they were written in the last 20 years or so - I read them about 5 years ago.
Our story starts with our hero (an ex-SAS soldier) fighting in an underground fight-club. He still has all his augmentations but they are turned off as is required when he demobbed.
He wins his fight, and goes home where you discover that Earth society is very unequal - the haves live in tall towers, while the have-nots live on the streets (our hero is a have-not).
He gets propositioned by an guy (who's like military intelligence, or part of a corporation or something) to go and rescue or find....errr... something. He eventually agrees, and gets his augmentations switched back on.
The rest of the book is him doing this mission, and ending up with a bit of a team, but by the end he finds that the grey-goo aliens they've been fighting aren't aliens, but are actually created by the corporation.
Oh, and he gets really, really badly injured at the end of the book but somehow survives (at the end of book 2 you find out that's not actually true). The book ends with him and his team in a broken "alien" spaceship kinda just drifting in space.
They fix the spaceship and head back to Earth, having various adventures along the way. The one bit that stands out is he has to fight a guy (ex-Delta Force or something) with a mono-filament sword or whip or similar, that will cut through pretty-much anything. This guy joins his team afterwards.
Stuff happens, they end up back at Earth, and I think they end up destroying a space-elevator (I can't remember why, but it's important). Right at the end you (and the hero) discover the hero actually died at the end of the first book and is basically made of grey-goo now. I think he actually dies at this point too.
Sooo, if anyone knows what this book is that would be amazing - I've spent a couple of months trying to find it again, but my google-fu is letting me down. :(