I saw this in my local Sainsbury's store (Haywards Heath) this evening.
They didn't appear to be reduced to clear, infact it was in the "Game of the Week" section for PC games so I would assume this offer is nationwide.
The next cheapest I can find it for is £16.99 (+ VAT) on Steam. Failing that, you are looking at approx. £17.99 from most online retailers.
Here are the games that are included within The Orange Box:
- Team Fortress 2
- Half Life 2: Episode Two
- Half Life 2: Episode One
- Half Life 2