If you have symptoms it’s not necessary to get a test. You should stay at home
This is the exact opposite of the advice which was changed when mass testing was brought in. In April this was the advice because they did not have capacity now they do.
It is vital to get a test in order for the track and trace process to work and ensure that any close contacts also isolate as otherwise its allowed to spread into the community for a longer period of time. This is especially important for people who are asystomatic.
Any of the symptoms you should book a test for and isolate until the results if negative or the end of the isolation period if positive