I have a residence card as a family member of a Slovenian citizen, and we want to visit the UK with my husband (Slovenian citizen) do I need a visa?
If the residence card explicitly says "family member of a union citizen", then it should work for entering the UK without applying for a visa or other permit in advance.
... That is, at least until the UK leaves the EU, which it is scheduled to do 4 weeks from now. What happens afterwards is, at the moment, anyone's guess.
The UK government has stated their intention that the current rules will stay in place for some transitional period after Brexit, but whether they will be able to deliver on that promise remains to be seen. There are political groups in the UK that would like nothing better than to abolish the rules you'd be depending on overnight, and it's looking increasingly possible that they may get their way.