Crewsaver are a top name in the marine world and I would expect to be warm for the day if I wore a drysuit like that for dinghy racing, so long as I was wearing the right stuff under it. The suit itself keeps you dry so the insulating layers you wear under it can keep you warm.
Will it be comfortable in the cockpit? Probably, but don't buy one that's too small. The cockpit in a 12' dinghy makes the cockpit in the average GA plane look exceedingly spacious, plus you are not running around it all the time.
However, if you end up in the Irish Sea in (say) February in poor conditions the water will be rough and cold, and finding you will be a challenge - you can stay with the plane as long as it floats (and I would say you should do - with it, not in it), but once it sinks you are an invisible pinprick. An inflatable lifejacket (Crewsaver again) might be a wise addition, plus maybe a hood to reduce the amount of spray you inhale. Think about how to make yourself easier to find - a personal beacon and maybe a lifejacket light might be prudent purchases.