There was a time when there were several of us on here with Cytech Comfort alarms. These fail the 'cheap' test - they never were cheap, but they're now significantly more expensive than they were when I installed a couple 10 years ago (BTW, both still working fine). They can do serial or ethernet connection via add-on cards. Software is available (Windows), but the raw serial/ethernet protocol is fully published so you can write your own (which is what I did).
In addition to the security alarm function, it's a fully featured home automation (HA) system. If you were only going to use it as an alarm, I'm guessing there are cheaper systems, but I'm way out of date with what's on the market now. If HA interests you, then you might consider the additional investment in a Comfort system worthwhile.
System is designed for either DIY or professional install, but you will need some knowledge of programming to configure it. The programming is extremely powerful and you can use that to do pretty much anything you can think of.
It can be programmed to conform to UK requirements for monitoring with police response, but would need to be on maintenance contract with your ability to program it locked out. Otherwise you can have it monitored without police response, or unmonitored, and have programming access.