There are generally two types of cover for this.

An office would often take out cover for "Increased Costs of Working", this would cover the additional costs to continue trading eg renting out a temporary office so they could continue trading and / or the cots of diverting calls etc etc.

A shop or manufacturer (Basically a business that cannot just up sticks and trade from anywhere would often have "Business Interruption Cover". This would normally cover them for the their loss of either net or gross profit caused by the claim.

The first might cover extra costs for staff eg if they had to pay overtime or extra travelling costs or hire extra staff to contine trading but would not cover the normal staff wages as these would have to be paid anyway.

You should speak to a decent local broker who can assess your businesses own requirements and what is best

Don't quite know what you mean by 'usual'. The type of insurance you are referring to is called 'consequencial loss' and is sometimes available as an added option to a businesses normal insurance cover. Whether its 'usual' or not, I don't really know because I don't know how many businesses would take up the option.

Would it be usual for a small business to have an insurance policy to cover them for loss of income and to compensate staff wages due to natural flooding which resulted in damage to the premises and loss of electricity meaning that the business had to close for a day?