Does a landlord have to have insurance has a short answer and a longer one that matters more. Short answer: only if you employ somebody. Longer answer: a mortgage, a lease and a letting agent will each require cover, and the liability exposure exists whether or not anybody requires anything.
The compulsory one
Employers liability, from the moment anybody works for you rather than for themselves, to the statutory minimum limit, with a penalty charged for each day you trade without it. A cleaner, a caretaker or a gardener working on your instruction usually counts; a genuinely self employed contractor carrying their own cover does not.
The effectively required ones
A buy to let mortgage requires buildings cover continuously with the lender's interest noted, and the lender can insure the property itself and charge you if it lapses. A flat's lease puts the structure on the freeholder and may require you to hold liability cover. A letting agent commonly requires liability at a stated minimum.
The one nobody requires and everybody should have
Property owners liability. It costs very little, it answers the largest possible claim, and the exposure is created by duties a landlord owes whether or not they are insured. Declining it is self insuring an open ended liability to save a small part of a premium.
Questions people ask about does a landlord have to have insurance
Is landlord insurance legally required?
Only employers liability, and only if you employ anybody. Nothing else is required by statute in the UK.
What does a mortgage require?
Buildings cover for full reinstatement against listed perils, held continuously, with the lender's interest noted.
What if I own the property outright and employ nobody?
Then nothing is required. Property owners liability is still the section worth having, because the exposure exists regardless.