A timely thread, Trev. I have been asking myself the same question and infact I was tempted to sign up for the Inverse Winter race, myself.
I know it will get laughed at by some on here but I have been watching a lot of You Tube vids of crits and saw a few of the Hillingdon ones. As best I can tell, the avg pace will be perhaps 25mph (ish) - which may not be quite as bad as it sounds (given that you will be closely following a wheel for the vast majority of the time). I decided that I will need to move up to the training group club rides and see if I get shelled out of the back, first. I will probably try 4th category racing in the spring, all being well but I need to improve my sprint and make sure I can sustain 20+ mph in a group for 1 hour+
Good luck and let us know what happens!