It is important to declare any high value items to your insurer, because your premiums and the amount of cover you have will be based upon this valuation of all the insured possessions. If you are under-insured, it's very unlikely that you'll get enough money from a payout to cover the cost of replacing or repairing a high value item.
High value items are things like:
Generally speaking, anything can be a high value item. Contents insurance providers generally set the benchmark at the item being worth over £1,000. If the sum total of all of your possessions is over £50,000, it is strongly advised that contents insurance for high value items is considered.
Many of this type of policy will come with a specific list of exclusions, so consumers should be careful to make sure the high value items that want to insure are definitely included in the contents insurance policy that they choose.