It is not just the CAT III ILS that would have to be put in, it is the rest of the lighting system which is the expensive bit. You also have to have another set of holding points marked everywhere (with stopbars) and so on. Then there are the approach light requirements, it is possible to provide approach lights that are flush mounted, but all very expensive.

You also have the problem of where to mount the ILS localiser antenna array. If you go for 08 end, then the antenna need to be mounted near the main ST pier. If you go for the 26 end then the localiser ends up against the noise bund which might give you problems. There are also obstacle issues to consider, although I have not got the obstacle data to see what the relevant obstacle clearance altitude would be. Then there is the glidepath antenna as well, not that this is technically difficult, just financially difficult.