I've copied and pasted from the government website:
You can return to your normal routine and stop self-isolating after 10 full days if your symptoms have gone, or if the only symptoms you have are a cough or anosmia, which can last for several weeks. If you still have a high temperature after 10 days or are otherwise unwell, stay at home and seek medical advice.
If you are isolating because of a positive test result but did not have any symptoms, and you develop COVID-19 symptoms within your isolation period, start a new 10 day isolation period by counting 10 full days from the day following your symptom onset.
If you develop COVID-19 symptoms at any point after ending your first period of isolation you and your household should follow the steps in this guidance again.
From another bit of the site:
If you develop symptoms while you are isolating,arrange to have a COVID-19PCRtest. If your test result is positive, follow the advice for people with COVID-19 to stay at home and start a further full 10 day isolation period. This begins when your symptoms started, regardless of where you are in your original 10 day isolation period. This means that your total isolation period will be longer than 10 days.
If other household members develop symptoms during this period, you do not need to isolate for longer than 10 days