Sounds to me like simply FTP'ing your data elsewhere might be the answer. Some alarm systems have a closing contact when they're armed or you could stick an opto-isolator across the alarmed led on your control panel. I doubt a regular home alarm system would have a serial port on it.

Sticking the server in the loft will probably just result in regularly clogged fan filters and unreliable running in summer. As you can see the bloke actually stole the machine taking the pictures but because it emailed the data off site they still got their evidence.

That said, when I got done over, the first thing they did before entering the house was to cut the power.