if it's being burried then i'd be inclined to use some braided cable and run into a jb with some choc block, then run it out with some 4/6 core alarm cable.
Main cable
SY Braided Cable
Jb
Plastic Junction Box
Alarm cable
Alarm Cable
and some choc blok and armour gland from b&q, all those are examples. you might think it's a bit ott but saves using small alarm that may get damaged and those bits should set you back no more than £30-40 max.