Most dealers will carry out valuations, but it depends on what you want to insure it for. If to cover replacement at cost, then it would obviously be necessary to provide evidence of that cost. If insured to current market values based on reference tomes, then a regular review of the insured sum will be required and bear in mind that could go both up and down from year to year. In terms of cost, that will depend pro-rata on the number of items to be valued as the time required for each coin would not be wildly different. In terms of cost, time is money, so an hourly rate based on the average wage plus expenses is a reasonable ballpark figure.
Insurance companies will allow you to change valuations during the year, usually accompanied by an adjustment in premium. Keeping images of the collection would be good for proof of ownership if recovering property, but a chocolate teapot in terms of proving that you paid £xxx for it. I offer receipts for sales at fairs, but less than 10% of buyers want one, You are not alone.
In terms of insurance premiums, the specialist insurers will generally want 0.5% of the sum insured as a premium. There's an element of swings and roundabouts in how the premiums are apportioned, but the total rarely diverges by much.