This is a sound that is enhanced by acoustics, and may also be an electronic sound (recorded from real horns) used on submarines and ships. Any 120 volt electromagnetic horn operating on 60 Hz. will sound like this, especially if it has a projector horn. The closest ones to this sound I can think of are horns by Simplex and Benjamin that have projectors. Vibratone-style square horns tend to be more raspy. Get any horn of this type and sound it in an unused gymnasium, and it will sound like this.
By the way, who in their right mind is going to shell out 4 quid (about $9) for a 25 second alarm sound? For not much more, you can buy the real thing on Ebay! Here's one that's considerably overpriced (there are three resonator horns for sale at half this price) but gives you an idea:
http://cgi.ebay.com/HORN-SPERTI-FARADAY ... 0084864439