My landlord or agent wants to inspect the property, arrange a viewing or carry out repairs
Tenants have the right to "quiet enjoyment" of their home. Apart from genuine emergencies, landlords cannot enter a tenant’s home without their consent unless they have a court order. If they do so, it could be considered harassment, which counts as a criminal offence.
Your landlord must give you at least 24 hours' notice in writing if they need access to your home. You can still refuse access (read more).
Viewings are permitted as of 13 May but the government has advised "If any member of either the household being viewed, or the household undertaking a viewing is showing symptoms of coronavirus or is self-isolating, then a physical viewing should be delayed."
Your landlord is still responsible for keeping the property safe and arranging for repairs to be carried out in a reasonable timeframe. If they are legally required to have an inspection - e.g. to renew the gas safety certificate - but you are self-isolating and wish to delay it, they can respect your wishes without risking prosecution (see 3.9 here).