An unusually reasonable and well presented question, but the usual mixture of good and bogus answers,
BOAC is mainly right. However, to the best of my knowledge, there are no Cat3c (no DH and no RVR limit) appoaches authorised. Even Cat3B with no decision height has a visibility (RVR) limit to enable safe clearance of the runway under manual control. Also, some aircraft are authorised for Cat2 approaches, but to a manual landing. So not all Cat2/3 approaches end in an autoland.
In the UK, subject to the airlines rules, any 'in line' ILS can be used in VMC for practise CAT2/3 appoaches to an autoland. You do not need to tell ATC (again, subject to your own airlines rules) since you will be able to monitor the quality of the guidance visually and can disconnect the automatics for a manual landing if the flight path errs. On a busy VMC day most ATC units will decline a request for them to provide Cat2/3 safe guarding due to time, manpower, and landing rate constraints. Some military ILSs are not 'in line' with the runway so that there is room for a jet barrier at the end of the runway where the ILS localiser aerials usually live. An autoland to such an 'offset' ILS would be technically possible, however, shortly after touch down the aircraft would gracefully steer itself off the side of the runway as it continued heading towards the localiser array. Indeed it may also land short of the other side of the runway. On any runway the autoland touchdown zone is a box rather than a point due to the inevitable tolerance in lateral and vertical guidance.
There are some Cat1 ILS installations that are technically CAT3 capable but for which the airport operator chooses not to go down the very expensive route of qualifying them for Cat2/3. There are also some Cat 1 installations that cannot generate a tight enough beam to deliver the Cat2/3 flight path tolerences. If you look at an ILS array, the fewer aeriels, the wider the beam. Don't quote me, but Cat 1 has something like 7 or 9, and Cat2/3 has many more. There are also ground lighting, beam protection, and physical security issues that conspire to put the bean counters off the idea of providing Cat2/3. The airport, the aircraft, the airmen, and the airline all require qualification and certification in order for Cat2/3 ops to be approved.
Under JAR (euro) regs the pilots can achieve and maintain currency in the sim. Nevertheless, as the same rules require the aircraft to be regulalrly checked, it results in practise cat2/3 approaches being conducted in VMC, although a real approach in fog would, of course, also count towards the aircraft monitoring programme.
Apart from the requirement for an 'in-line' ILS, there are, as already noted, a few reasons why some Cat1 runways are not suitable for practise autolond. At BRS (Bristol, EGGD), the upslope before the threshold and the 'hump' on the runway on 09 denies the aircraft system the chance to correctly calculate the flare. However, the other end (27) is Cat3 capable. At MAN (Manchester, EGCC), a steep valley on finals for 05L precludes Cat2 ops for large aircraft, but does allow Cat3, ironically, as the system (particularly the rad alt) can get it's act together for an autoland once the valley has been overflown.
Hope this helps
