On the subject of Cat 3 ILS, it's not particularly a matter of obstacle clearance but more to do with the extra ground facilities required, i.e runway lighting and additional holding points.
For example, at Leeds, the location and poor weather you encounter when you develop an airport at 628ft justifies the extra investment in facilities, whereas at HUY, NWI and others which are both just over 100ft above SL, the investment has not been worthwhile.
XL didn't pull from HUY because of a lack of Cat 3 ILS, it was because of lack of trade.
As for DSA, they have some way to go before they catch up with the progress that LBA have made, but at the rate they are going, they'll make 2million pax within the next 18 months, not through scheduled services, but via the long haul market which LBA cannot cater for.