Looks like the cold weather caught B&Q on the hop so they are selling off the units cheap, you do need to factor in the engineers time to gas up the system even if you do the instal yourself.
12000btu system also available for £174.50
Airforce offer a range of three professional fit climate control machines. These units can only be fitted by a professional installer who holds a refrigerant handling certificate and is competent with such installations. They are not intended for DIY fitting.