A private individual may find that an instrument is covered by their house contents policy as long as its value is below a designated amount. Two thousand pounds (£2,000) is quite a common figure, but your policy document should be checked. Above this value it may be necessary to obtain a written valuation which might entail paying a fee to a specialist valuer. See How can I get a valuation? In addition, instruments may not be covered if taken out of the house.
There are now a number of insurance companies which specialise in providing cover for musical instruments. Many of them can be found on line by entering musical instrument insurance into a search.
Please note that MIRN does not endorse any particular insurance company.