I believe slots at the F18 worlds are based around membership numbers of the NA, also the UK allocation looks wrong as I think it is 20. The numbers can be a bit misleading because it is paid up class members which can include crews.
With regard to F16 numbers I would say that in the UK there is a perception that F16's are not competitive in respect of their handicap. The Spitfire guys certainly seem to think the Spit is quicker yet gains a good deal of time as well against the F16. Likewise with the F18 that sails off the same number (I think). I've not raced (F18) in light airs against an F16; but a couple of times in reasonable wind at the Mumbles last year to my mind the F16's didn't appear as quick as the F18 (also looked overpowered one up - sorry Mark <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" />). I would suggest that maybe a JP type figure needs to sail the boat at opens and get some top results and then people would be more interested, certainly in the racing sphere.