People in this country (UK) are often do not know what is best for their cat, to keep her indoors or let her roam outside. Especially since there is this general belief that if you keep your cat indoors is really cruel, as the cat should be left alone to explore the outside world. WRONG!
In the US most of cat owners keep their companions indoors as it is the easiest way to extend the cat's life, for many simple reasons; Indoor cats that eat well often live to almost 20 years and more. On the other hand outdoor cats have a shorter life span due to road accidents, disease and injury.
Turning your cat into an indoor pussycat it's not going to be easy but as long you provide her with stimulation, play, attention and rewards that will substitute for what she was getting outdoors, she eventually get used to to an easy life. Make sure that the cat(s) have good scratching posts (the ideal one should be in the form of a multi-platform cat tree, which your cat will love and it will save your sofa, chairs and speakers), also cat toys or even movies for cats are all good ways to keep the cat stimulated.
When you are home you also provide her with extra petting and grooming as this is another way to keep a cat happy indoors.
The most important thing is that the owner should have a lot of patience and determination and should remember that this is for the good of the cat and to avoid serious injuries and diseases as cats can easily catch dangerous illnesses from stray cats.
You should expect of course a lot a fuss from your cat, a lot of meowing and scratching at the door and windows while the transition takes place. NEVER EVER GIVE IN. BE PATIENT!
Give your cat access to several good window spots and provide her with extra toys, catnip, edible grass etc., as everything helps. In about 2 weeks or less cats usually adjust. Also don't believe that because you have a high fence your cat would never leave your garden, they can jump at least 3 times their height and stray cats would also find a way to invade your cat's private space. Also don't let the cat alone on the balcony as they're known to jump and being able to find ways to 'escape'. Don't give in and don't think that you are being cruel, it's for the good of your cat!