You'll need "Furnished Holiday Let" Insurance. Shop around, prices differ widely. Some names are Boshers, Schofields and i4me. Unfortunately your current insurance is invalid! The main difference is the amount of Public Liability required - this should be £5 million for a sleeps 4 house.
With regard to planning, do check with your local council as requirements differ, we checked and our council confirmed we did not need any planning, but different councils interpret the planning reqs differently (with more of a hard-line in traditionally touristy areas).
As you aren't resident in the property, you'll have to change from Council Tax to Business rates. The good news is that if this is your only business, it's likely you'll be eligible for up to 100% business rate relief. The bad news is that councils do not collect, or charge to collect rubbish for businesses, so you'll need to pay for licensed rubbish disposal (its illegal for commercial rubbish to enter the domestic rubbish cycle).
Yes to all the things mentioned re. smoke, CO2, fire blankets. Our holiday let insurance also stipulates the number and type of fire extinguishers on each floor, and how many days the property can be left empty without taking the precaution of draining the heating system. Also make sure your fire risk assessment, safety risk assessment, electrical testing, boiler servicing, chimney sweeping etc are all up to date, as these could invalidate insurance in the event of a claim.