This is a very common question. But the answer is not so simple.
The UK Gambling Commission has been very clear that operators need to have a UK license in order to be able to advertise in the UK. However, there are some gray areas in which an operator may not have a license but still be able to advertise in the UK because they have an affiliate with a UK license and can share their gambling licence with them.
The bottom line is that all new casino sites need to be licensed by the UK Gambling Commission and should always check this before you sign up for any new casino site. Also you can read this article: https://pressfrom.info/uk/news/tech-science/-561855-do-all-new-casino-sites-have-a-uk-license.html