What expenses can I claim as a landlord?

Costs incurred wholly and exclusively for renting the property out, and repairs but not improvements.

The usual list is letting agent fees, ground rent and service charges, buildings and contents insurance, accountancy, safety certificates, and general maintenance. Replacing a broken boiler with a similar one is a repair. Putting in a new kitchen where there was a serviceable one is an improvement, and improvements are dealt with when you sell rather than each year.

Mortgage interest is treated differently from other costs and has been since the rules changed. That one is worth reading about separately.