The following characteristics are protected characteristics—
marriage and civil partnership;
pregnancy and maternity;
religion or belief;
These are the current grounds upon which it is illegal to "discriminate", i.e. differentiate between people. So for example if you want to interact with a person in some way it is forbidden that you decide who, and how, according to any of those characteristics.
The standard example, giving someone a job. You may decide who to hire, and in what manner or for what job, according to a range of personal attributes excluding protected characteristics, which must not enter into the decision making process.
In less formal settings, it's also extremely dangerous to criticise or joke about any of these characteristics, and there are punishments available for those who do.
So, I'm wondering what other characteristics we think should be protected? I can think of lots of things that today people think are acceptable to discriminate against people because of. For example, hair colour, height, general appearance, wealth, various possessions, what job they have, all of their hobbies and interests, figure and physique, accent, "intelligence", etc.
For a fair and respectful society I think the current list of protected characteristics, while a good starting point, does not go nearly far enough.