The following is a work in progress. It's a list of what you will need, but at some point I will list out the stuff that we have successfully got working, and we may even sell it.
Your alarm system will consist of Devices to send data to MyAlarms.co.uk, and devices to receive instructions to make something happen (such as sounding a siren or flashing a light).
You will need to buy ESP8266 modules, plastic boxes, magnetic alarm triggers, batteries and battery holders and some wire.
You will need to buy ESP8266 modules, relays, 12 Volt Car Batteries and plastic project boxes and some wire. The device will trigger the relay, which will then provide power to whatever it is that you want to power.