In my reasonably limited experience local fell races tend to be the cheapest and most informally managed, with a high degree of self-reliance and self-navigation/route selection required for a lot of races. This makes them a lot of fun.
Longer trail events are largely commercial in nature, pandering to those who want t-shirt, medal etc., but reflecting the need for providing insurance etc. I did the Country to Capital ultra yesterday which was self-navigation/no routemarking/map provided, with interim check points. However, we were provided with geotrackers and this is common where it'll take runners 8 or more hours in the interests of safety.
I find the running Twitter community very helpful and friendly (#UKrunchat is a good place to start) and you can always put a shout out suggesting a route and time/place to meet and hopefully someone will turn up. I don't tend to "enter" many events, and fill the gaps with DIY adventures. I have a couple of solo ultra's (company welcome!) that I'd like to do where I can get a train back to the start afterwards.