Not the cheapest deal on this but the lowest price in stock at the moment and to get it for for under £20 is worth it.
The Orange Box includes all the content of The Black Box, plus the original Half-Life 2 and Half-Life 2: Episode One.
Half-Life 2: Episode Two -- the second installment in Valves episodic trilogy advances the award-winning story, leading the player to new locations outside of City 17.
Portal -- a pioneering type of single player action game that rewrites the rules for how players approach and manipulate their environment much like how Half-Life 2s Gravity Gun reinvented the way gamers interact with objects in the game.
Team Fortress 2 -- an all-new version of the legendary title that spawned team based multiplayer action games. The games daring new art style features the most advanced graphics of any Source-based game released to date.