Two things govern when you should take your test:
You should only take your test on a day when the Royal Mail can collect your samples. That is, make sure you can put them in a Royal Mail postbox (or take them to a post office) on a day and at a time when they can be collected that same day (i.e. not a Saturday, Sunday, or bank holiday).
If you happen to catch an STI, it takes a varying amount of time from when you come into contact with the STI and when a laboratory test can detect it. This is called the "window period" -- which is the time between host exposure to an infectious agent, and when a test can accurately detect the presence or absence of that infectious agent.
Fore more information on window periods -- i.e. when a test can detect an STI -- please refer to this helpful article:
https://yoxly.com/en-GB/blogs/yoxly-news/your-guide-to-incubation-latency-and-window-periods