They're pretty compact/slim. I've used the same pair on a singlespeed commuter for two years and an old Trek before that and have been very happy with them. Clearance under the fork crown will depend on how chunky a tyre you run and appreciate my experience was not with an S2. Looks like some positive experiences on this Cervelo forum:
Another option might be to strap something to the downtube as a partial solution. Instant air brake:
Good luck with the ride. lovely bit of the country.
You could fairly easily DIY a bracket like the one below. Mount it to the fork brake you could bond to half a fender for use over the top of the wheel and use downtube mounted guard for a little protection there.
http://www.zefal.com/en/mudguards-prote ... -fc50.html