This comment is about 24h TTs but it may well apply to 12h.
Just to show the extent to which 24h TTs are something of a backwater event, there's only one per year in the UK, the National 24h championships . Some years ago I wrote to the organisers to ask if there were lower level 24h events that I could have a bash at - I couldn't find any and obviously it would be ludicrous for someone like me to be in a National Championships event. They replied that theirs was the only one, and that I would be very welcome.
When an event has so few entrants that a random joe bloggs with no history is told by the organisers (I quote) "I suspect you won't be out of place because there is a huge range of abilities." then you know that it's pretty far from being the gold standard. Imagine if I wrote to M Prudhomme and said "I fancy having a bash at riding round France, it looks like a laugh".
I checked on the CTT website and there is indeed only one 24h event and there seems to be about 3 12hr events.