HMV are selling "ARMA: Armed Assault" for £7.99 and "ARMA: Armed Assault - Gold Edition" for £14.99 for the PC (inc Del.)
You can get cashback through the usual sites (e.g. 5% Quidoc, making them £7.59 & £14.24)
The next best price seems to be £12.99 (Gamestation) and £17.99 for the Gold Edition (Amazon)
The Gold Edition features both the original game and the Queens Gambit, Expansion Pack.