Clifford FTW
Used to have a Clifford CAT 1 with proxies, tilt sensor, glass break, anti-car jack etc on my old Corsa, was about £400 fitted IIRC. Most decent alarm systems you can only buy including fitting IIRC.
You can buy kits to do it yourself but I really wouldn't want to risk it tbh, if it all goes wrong you're goosed.