I found this on the government website but I can't find anything anywhere about what you do if your test is negative. I'm sure it used to say you could stop isolation. The NHS website says you can (see below)
Gov.uk
Ending self-isolation
If you have had symptoms of COVID-19 or a positive test, then you may end your self-isolation after 10 days and return to your normal routine if you do not have symptoms other than cough or loss of sense of smell or taste. If you still have a high temperature, keep self-isolating until your temperature returns to normal and seek medical advice.
After 10 days, if you just have a cough or a loss of, or change in, your normal sense of taste or smell (anosmia), you do not need to continue to self-isolate. This is because a cough or anosmia can last for several weeks once the infection has gone. The 10-day period starts from the day when you first became ill.
If you continue to feel unwell and have not already sought medical advice, you should use the NHS 111 online COVID-19 service. If you do not have internet access, call NHS 111. For a medical emergency dial 999.
NHS
Negative test result
A negative result means the test did not find coronavirus.
You do not need to self-isolate if your test is negative, as long as:
everyone you live with who has symptoms tests negative
everyone in your support bubble who has symptoms tests negative
you were not told to self-isolate for 14 days by NHS Test and Trace – if you were, see what to do if you've been told you've been in contact with someone who has coronavirus
you feel well – if you feel unwell, stay at home until you’re feeling better
If you have diarrhoea or you’re being sick, stay at home until 48 hours after they've stopped.