I’ve never used a licence for these Enduro type events like the Innerleithen day/night things or 24/12/10 hour races.
I did have a BC license for my first year of XC racing but it cost me £80 including BC membership and it turned out in some classes you didn’t need it anyway.
As for DH, I’m not sure about the National DH series but for things like the Alpine Winter series you shouldn’t need one. Best to ask the organisers themselves though- you’ll get many different answers on here and the only people qualified to give you the right one are the people putting the race on.
Ask while you’re talking to them if you can use a TLI license. These are much cheaper than a BC one with all the same benefits. If you can use these then get one. It’s set up by roadies but it gives you a race licence and I have one that I used to organise an MTB race last year.
tlicycling.org.uk/
You need points to enter some races, but I’m not sure which. It’s usually UCI category races and most races you should be fine, but again, it’s best to ask the individual organiser.