Historically it was an IM event. There's been three breaks from that; Challenge Roth, Outlaw and Challenge Henley. I don't think it has to be an IM event anymore, but those events tend to prove the most popular. There tends to be a drive towards it being abroad. The weather in the UK is considered too unreliable.
When we did it in 2013 the practice swims we did in the sea two days before were the first OW swims that year for many of us. I'm not sure how a novice overcomes that issue. Aside from getting very cold! I think we just bit the bullet and recognised that most of us had swum OW in previous events. Again, not much help for the novice pirate.
Mallorca is September, I think, with a cheeky half in May.
Thinking about the novice issue some more, an observation might be that there is a big pirate following at Outlaw for novices. These days, lots of folk seem to start with Outlaw and work up from there.