Don't think TF2 or CSS is any good for you because you sound like you like missions and a linear & scripted gameplay...
I
highly recommend :
Half Life 2 ( a must have game, any first person shooter fan must have tryed this, no excuses, it has a great story, has a very fun gameplay and is more than your ordinary shooter, gfx look quite well too ( better than a lot of much newer games)
Half life 2 episodes 1&2 if you liked the original hl2... ( same as half life 2 really, but sequels to the awesome story).
Far Cry ( just awesome, first fight mercenary's in a beautiful tropical environment and later you fight mutants in dark places, the rest of the game is a mix of the 2, it's quite tough even on easy and imo also a must play, since you enjoyed halo I doubt you'll hate the mutants, gfx still look great to this day, it's only 6 euros at retailers here so I'm sure you can get it for 5 quid, just do it !
Portal: It's a first person puzzle game really, you shoot a portal with your portal gun and progress through the game, except (mostly) trolls I haven't found anyone that hated it, all the people that finished it think it's awesome. Imo this game has one of the best humor ever in all games, it has a great story and is a must have played game really, a bit short though but once you get to the last 5 or so test chambers it's just awesome.
And also recommend:
Bioshock: An awesome shooter with a good story, but it's a bit cartoony although beautiful ( it's an underwater city), try the demo first perhaps.
Original
Medal of Honor allied assault : Dated gfx but still quite good to play, has good missions.
Doom3/Quake 4: Not the best story's, but it's no ''mp'' game like battlefield, you follow a single player story with missions. Doom 3 is more of a game where you play on your own against mutants etc. Quake 4 is more squad based and less ''dark/scary'.
FEAR: Good gfx, okay ish story but a bit repeative, still very cool to try, make sure to try the demo if there is one...
There are a few more but I'm tired and can't remember em atm

, but really check out the games I mentioned first

...