I’d avoid the out and out XC races (well i do anyway) I enjoyed a lot of marathon events last year, some competitive most not officially races. You are right to say it is mostly about riding a marked course with a good group of people, but I suspect like Road Sportives that things at the front of the field get a bit more serious. Results are almost always published online.
The CRC marathon series is well worth a look, the Afan Monster was my favorite event by far last year which was for charity and even the Evans Ride It! I did in the Cotswolds was fun.
As you say they I think the format is very similar to Sportive. Also look for anything with Randonnée in the title, I did the Cheddar one last year but looks like they cancelled that one this year.
I’ve put together a list of things I’m interested in here http://gothandy.wordpress.com/events/ be warned there will be a Bristol bias to my list.