Ground equipment:
cat1 and other cat landings ILS is the same except for the failsave items (even if one component of the system breaks down or is outside the set parameters, there should be a backup system-component) and those differ from cat II and III. then you have to have signal integrity; no disturbance allowed, it might 'bend' the localiser beam or the glide-slope. so you need a interference free area. But the terain surrounding the threshhold is of great importance too!(i beleive as said above.)
for an aircraft it is not very hard to become catII but catIII is much more costly and involves systems only installed on larger jets. for small commuter planes lets say up to 70 seats the investment is too high regarding theire yeild.
so as jersey is mostly visited in the summer and mostly by a limmited size A/C, and the rwy is not of the flat kind......you can gues its too........