Most office insurance policies are designed to be all-encompassing and to provide office based businesses, with the range of covers they will need.
You will normally have certain sections of cover which are included as standard. Removing them does not alter the cover, usually these are money, business interruption, public and employers liability.
In addition to these, there are other sections which you can choose to add if required. The two main types are for buildings and equipment cover away from the premises (such as laptops and smart phones).
The buildings cover is self explanatory. If you own, or have full insuring and repairing lease, then you will need to have a separate section covering the office premises.
But, if you lease the premises, you may choose to make adaptions, adjustments and improvements to the building itself. This may cover, putting in new toilets, partition walls, false ceilings or high tech wiring. If you chose to move away, you would not normally take this with you as they are all improvements to the building.
If there was a fire though, then the cost of replacing these items does not fall under the landlords insurance, it is your responsibility as you have the financial interest or financial ownership.
This is why you have a secondary type of buildings cover, for tenants improvements. It is much cheaper to insurer than your standard office contents, because it is less likely to suffer a loss.
Just sit back and consider if you need to insure any tenants improvements. A good starting point is you lease, it should specify in there what you are responsible for. If you have spent money in these areas, you need to check that your business insurance policy has cover for this. Typically, for £10,000 of tenants improvements, it will only cost you around £30 a year to add to your policy.