SC gets you to see secret and occasional access to top secret. And a fetching yellow strip on your CS ID card It’s a desktop exercise. I had it for years ( and a more bespoke financial one ) despite my parents spending their 20s working for the revolution and my dad his 30s working for what was a front for Soviet military intelligence- ironically given he and my mum had been a major Euro communist and anti tankies in their 20s.
DV, they don’t seem to care what you do as long as you tell them about it- ISIS membership may be an exception. You get a green strip on your ID badge.
At the other end most civil servants just have a Basic check red or CTC check blue. The basic check just makes sure you aren’t actually wanted ( I think ) and the blue the same and that you aren’t actively on a watch list. Both give occasional access to secret. I think the CTC gives you access to a few of the more secure buildings.
Nb above may not actually reflect HMG policy.