I have always gone ribbing through the winter before on the basis that a) you get some lovely bright crisp sunny days b) its blissfully uncrowded and c) I've too much cash tied up in the RIB to have it sitting idle for 4-5 months of the year!!
Only reason for not doing it this winter is for max recovery time for the back problems I suffered in the summer.
Flanker has a good point about survivability in cold water. IMHO a drysuit with plenty of layers underneath is a must, even if its bright sunny and calm. If you end up in the drink, which can happen even on calm days, then you will need all the insulation you've got. (And yes like Wavehumper I have a certain amount of "personal" insulation which would help
). A decent wooly hat, fleece balaclava and goggles are also neccessary as is a pair of gloves. Think if you check back in the the forum this has been debated before but I use Gill Helmsmens gloves which are warm and waterproof.