If I were you and since I have played most of them I would say get the following:
TC's R6V. Excellent game for what it is. I played through the game storyline several times and it also has a skirmish mode, how cool is that! The sequel is good too and has some good refinements, but IMO, the first Vegas is the most fun. If you like FPS games, you should like it. I know it is still my favourite FPS of modern times, which says a lot for me.
GTAIV is definitely a good game and well worth 5 pounds. It's not my favourite game or anything, but it is hard to see how one could feel bad about buying it for a fiver.
(I miss the UK a lot)
Battlefield Bad Company (the first one I presume), is a good shooter and again, worth the £5. The terrain destruction is not as extensive as the sequel, but the game is still very fun to play and one of the very very few games I don't find the voiceovers annoying. They are even funny.
If you are not going to buy the latest version of Lego Indiana Jones, then the original ADventures is a good game to get. It does become better if you really enjoy Indiana Jones, but in general it is a fine game and as with all the Lego games of the last few years, they are solid entertainment, if you like this type of games.
I am still waiting for a Lego Star Trek game.
Regarding G-Force....there was a G-Force game in the 90's on the PC. Is it the same? If yes, then it is a good game, but it probably isn't the same one.