Just so you know ... there is a owner who has been trying to introduce two bengals for months now. When it gets this time consuming, it is NOT for the faint of heart. Many owners get another bengal for themselves and others feel their sole bengal needs a friend. Just as some kids or siblings can't get along, and we adult humans aren't friends with every single other human, cats have to find their friends, too. Trying to force a friendship is going to take a while.
Jackson Galaxy has some holistic remedies on his website that are for various issues and maybe something would work for you. Some people say they work, others say they don't. I don't know about shipping overseas. Feliway diffusers might help around the house, too. It's easy obtained at a pet store or through your Amazon in the UK.
The other thing is IT HAS ONLY BEEN ONE WEEK. You cannot expect much in a week, or maybe even two weeks. All you can do is take each hour at a time, day at a time and try to work some magic on the situation. I know it is wearing on you, but getting Luna and Nebula to co-exist peacefully is going to take time. And the issue seems to be with your older kitty, Luna. She is not happy with the youngster infringing on her space. Thus, keeping them separated most of the time and supervising any close interaction is what is required by you.
I'm just hoping you're not thinking, "gosh, I don't know if I can take much more of this." Ones decision to get a cat becomes a lifetime commitment. Gads, there are times I can't tolerate my 12-year old bengal. Especially when he is yowling in the middle of the night and wakes me up -- or I should say yowling several times during the night waking me up. But I love him and I made a commitment to put up with his quirks forever. We also have a DSH and while most of the time they are fine together, Raiden decides he wants to attack him. Happened twice this morning. So, you aren't the ONLY bengal cat owner in this world having issues with behavior.
Be patient, know that things can only improve from here. It is just going to take some time.