You will have seen them "ALARMS £155.00, £175.00". Do not be taken in....
Here are some of the reasons why they might be so cheap,
Some installers are illegally activating service timers to stop your alarm from working, so you pay a cheap price for the installation and the installer makes very little money, but when the timer stops the alarm from working the installer will charge you for that fault and now he starts to make money on your installation, he will then reset the timer and do the same again. In other words over time it is not a cheap installation at all.
Some installers will miss key features off the alarm to save money, a classic example is the external bell box, it is supposed to be tamper proofed so that if someone attempts to disable the external bell box the system will alert you inside the property. Rogue installers do not include this feature to save equipment costs but this means an intruder can disable the alarm before breaking in!!!!!!!! The rogue installer relies on the fact that the average customer will not go up the ladders to find out.
DIY stores alarm systems. Some can work fine but be warned, this is not usually universal alarm equipment. You pay for a full system then a couple of years down the line you get a faulty detector, when you try to replace it you find the product has been withdrawn and now you have to replace the whole system, its a common trick that means over 10 years you will pay far more than if you had a good system in the first place.