Hi
Sorry to say this, but I have sympathy for the position of the Insurance company. You will appreciate that Insurance companies are subject to millions of pounds worth of fraudulent claims, so they will deal with claims as you have outlined. Your claimed event may well be genuine, but you have to put yourself in the shoes of your Insurers. If it was your money being paid out, I would suspect you would have inspected the items very carefully. The people that inspect the items do this on a daily basis, so they should know what they are doing.
All you can do in this situation, is get the items independently inspected yourself and if the reports are in your favour, submit these to Insurers to enable them to reconsider. If they still refuse to settle, you could ask them to issue their final response, so that you could refer the matter to the Financial Ombudsman Service (FOS). The FOS wouldn't be able to look into this properly unless you have obtained the independent reports and have given your Insurers the chance to reconsider. Please be aware that even if you obtained independent reports in your favour, there is no guarantee the FOS would rule in your favour and it could take months for the FOS to issue their decision.
In your shoes I would look into how much it would cost to get a local company to inspect the items and provide a report to decide whether to bother or not. In my opinion, making a complaint to the Insurance company without any contrary professional independent opinion, is unlkely to succeed.
Huckster