Not being funny mate, but a ?100 alarm won't be worth a w*nk.
Seriously consider either saving for something better, or do what others have suggested, and pay monthly for a quality item.
You might not need a Thatcham approved alarm, but it will will be an indicator as to how good it is, and I wouldn't waste ?100 on a non-approved alarm when ?200 will get you on the way to getting a cat 1 jobbie.
Those cheap, self-install jobbies are garbage and aren't worth the time wasted on installing them.